Chapter 22: The Endocrine System

This chapter on endocrine disorders is divided into separate discussions of each of the endocrine glands. Each section begins with an overview of the physiology and biochemistry of the relevant hormones. In addition, because of the large number of (often complex) laboratory tests in endocrinology, each section has a brief description of the laboratory tests most frequently used to diagnose the disorders in that disease group. Tests for which either serum or plasma is an acceptable specimen for analysis are noted as serum tests. Tests specifically requiring plasma are indicated by inclusion of the word “plasma” before the test name. As with all other chapters, each disorder is presented with a description of the disease and information useful in establishing a diagnosis. Figure 22–1 shows a general approach to the patient with endocrine disease.

Physiology and Biochemistry

Production of thyroid hormones is regulated by the hypothalamic–pituitary–thyroid axis (Figure 22–2). Thyrotropin-releasing hormone (TRH) is produced in the hypothalamus and induces thyroid-stimulating hormone (TSH or thyrotropin) production in the anterior pituitary. TSH, in turn, stimulates thyroid hormone production and release by the thyroid gland. TSH production is inversely related to plasma thyroxine (T₄) and triiodothyronine (T₃) concentrations. The 2 primary hormones synthesized and secreted by the thyroid gland are T₄ and, in lesser quantities, T₃ (Figure 22–3). They are transported by plasma proteins—notably thyroid-binding globulin (TBG), transthyretin, and albumin—to various tissue sites where T₄ is deiodinated to the active form, T₃, and the inactive form known as reverse T₃ (rT₃). Thyroid hormones act through nuclear hormone receptors that are transcription factors for numerous genes. These genes regulate a number of critical physiologic functions in development and metabolism.

Laboratory Tests

TSH

A “generational” classification has been applied for TSH immunoassays based on the assay sensitivity. Third-generation assays can accurately measure TSH as low as 0.01 mU/L. This allows the physician to distinguish mildly subnormal TSH values from the low values of overt hyperthyroid patients. The third-generation tests are also useful for evaluating the effectiveness of the thyroid hormone replacement in hypothyroid patients. Third-generation assays are essential for monitoring TSH suppression therapy in patients with a TSH-responsive thyroid tumor.

The relationship between TSH and the thyroid hormones, particularly free T₄, is an inverse log-linear one, such that very small changes in free T₄ result in large changes in TSH. Thus, TSH is the most sensitive first-line screening test for suspected thyroid abnormalities. If the TSH is within the normal reference range, no further testing is performed. If the TSH is outside of the reference range, a free T₄ is obtained.

Total Thyroid Hormone Measurements

Assays are available for both total T₄ and total T₃ measurements. These assays are quite specific and suffer little interference. However, transient changes in serum thyroid hormone-binding protein concentrations may affect total T₄ and T₃ concentrations. Therefore, an assessment for free T₄ (see below) is usually more helpful in evaluating thyroid function. The concentration of T₄ in the blood is usually 100 to 200 times greater than the T₃ level. In hyperthyroidism, total T₃ and T₄ concentrations correlate in all but a small subset of patients who have an elevation only in T₃. For that reason, T₃ should be measured in the serum of patients clinically suspected to be hyperthyroid and who have normal concentrations of serum T₄. Measurement of T₃ concentrations has limited clinical utility in assessment of hypothyroidism. Significant decreases in total T₃ are seen in euthyroid sick syndrome (ETS).

Free Thyroid Hormones and Thyroid Hormone-binding Capacity

“Direct” free thyroid hormone assays, without the need for a preliminary step to separate free hormones from hormones bound to protein carriers, are available for measurement of free T₄ and free T₃. Only a small fraction of T₄ (<0.1%) circulates unbound to proteins, and this has made the accurate quantitation of free T₄ analytically difficult. Free T₄ is a better indicator of thyroid status than total T₄ because, as noted above, the total T₄ is altered by changes in the amounts of TBG, albumin, and other thyroid hormone-binding proteins. About 0.3% of T₃ circulates as free T₃. In general, free T₃ concentrations correlate well with total T₃, but as with T₄, the concentrations of thyroid hormone-binding proteins influence the total T₃ level.

Although the free T₄ index offers an approximation of free T₄, this measure is largely obsolete due to improvements in the free hormone assays. The free T₄ index is calculated by multiplying the total T₄ by the measure of available hormone-binding sites on TBG in an assay known as the percent T₃ uptake. Because of the dependence on the amount of TBG, the free T₄ index and the T₃ uptake are affected by changes in the amount of thyroid-binding proteins induced by a variety of stimuli.

Reverse Triiodothyronine

Under acute stress or in illness, there is a shift in the T₄ deiodination in favor of the inactive rT₃ form rather than the active T₃. Numerous immunoassays are available for the measurement of rT₃ using antisera that do not cross-react significantly with T₃ or T₄. rT₃ is markedly increased in ETS syndrome (see below), but its measurement is rarely required for this diagnosis because its increase is proportional to the decrease in T₃.

Antithyroid Antibodies

Antithyroid antibodies are present in approximately 15% of the general population and are the most common cause of thyroid disease in iodine-replete societies. They are also present in selected autoimmune diseases not usually associated with thyroid dysfunction. Descriptions of 3 types of antithyroid antibodies follow:

• Antimicrosomal/antithyroid peroxidase antibodies (anti-TPO)—These antibodies are directed against a protein component of thyroid microsomes, the enzyme TPO. They are present in almost all patients with Hashimoto thyroiditis, in about 85% of patients with Graves disease (both discussed below), and in some patients with type 1 insulin-dependent diabetes mellitus, celiac disease, and Addison disease. An elevated titer of anti-TPO antibodies in the context of clinical symptoms of thyroid dysfunction and abnormal TSH and free T₄ results is diagnostic for autoimmune thyroid disease. The presence of TPO antibodies before or during pregnancy is a good predictor of those women who will develop postpartum thyroid disease (discussed below). Normal concentrations are not well established because the antibodies may be found in healthy people (up to 12% of the population), and the reference range depends on the method used to perform the test.

• Antithyroglobulin antibodies—These are also called colloidal antibodies. They are present in more than 85% of patients with Hashimoto thyroiditis and in more than 30% of patients with Graves disease. Like anti-TPO, antithyroglobulin antibodies also may be found in other autoimmune diseases. In iodine-sufficient areas, the antithyroglobulin antibody test is used less often, in favor of anti-TPO. However, in patients with suspected iodine deficiency, antithyroglobulin antibody is a better indicator of autoimmune thyroid disease.

• TSH receptor antibodies—These are a diverse group of immunoglobulins that bind to TSH receptors and influence their action. They are found in most patients with Graves disease and in patients with selected other autoimmune disorders involving the thyroid. The biological functions of these antibodies vary from thyroid stimulation to thyroid inhibition (by blocking stimulation induced by TSH). Antibodies referred to as thyroid-stimulating immunoglobulins are present in 95% of patients with untreated Graves disease. In vitro bioassays can assess the ability of stimulatory antibodies to induce functional responses in cultured cells by measuring cyclic adenosine monophosphate increases or adenylate cyclase activity. Assays are available that measure the capability of the inhibitory antibodies, called thyrotropin-binding inhibitory immunoglobulins, to block the binding of labeled TSH to its receptors.

Radioactive Iodine Uptake and Thyroid Scans

The radioactive iodine uptake and thyroid scan measurements involve the in vivo administration of radioactive iodine. Accumulated radioactivity in the thyroid is measured at intervals within 24 hours using a gamma scintillation counter. A nuclear imaging scan that examines the anatomic distribution of radioactive iodine uptake within the thyroid gland also may be obtained. Radioactive iodine uptake studies may be helpful when the diagnosis is in question and in differentiating between possible causes of hyperthyroidism.

Thyroglobulin

Thyroglobulin is stored in the follicular colloid of the thyroid as a prohormone. Thyroglobulin measurements are used to monitor treatment in thyroid cancer. Persistent serum thyroglobulin after thyroidectomy is evidence of incomplete ablation or metastatic thyroid cancer. Thyroglobulin concentrations should always be assessed in the context of an antithyroglobulin antibody test because these autoantibodies can cause false-positive or negative thyroglobulin results.

Calcitonin

Calcitonin is a polypeptide hormone produced by the thyroidal C cells, whose primary function is in regulating calcium homeostasis. It is elevated in patients with C-cell hyperplasia and medullary thyroid carcinoma (MTC). Calcitonin measurements are used to determine when to perform prophylactic thyroidectomy on patients with familial MTC. In addition, it is used to assess prognosis and monitor recurrence of MTC post thyroidectomy.

Fine Needle Aspiration (FNA) Cytology

FNA is the procedure of choice to collect a specimen for microscopic review to distinguish benign from malignant thyroid nodules. The sensitivity of thyroid FNA for detection of thyroid cancer and other disorders varies from 70% to 97%. It depends greatly on the quality of the specimens and the experience of the cytopathologist. Thyroglobulin can also be measured in lymph node FNA aspirates to diagnose and monitor thyroid cancer.

Description and Diagnosis

Hyperthyroidism, also known as thyrotoxicosis, is a collection of disorders associated with excess thyroid hormone (Table 22–1). There are 4 main causes of hyperthyroidism: 1) overstimulation of the thyroid (elevated TSH, human chorionic gonadotropin [hCG], and/or TSH receptor autoantibodies [TRAbs]); 2) genetic mutations leading to increased synthesis and secretion of thyroid hormone (germline, sporadic, or tumor induced); 3) release of excess hormone from the thyroid (inflammation, infection, injury); 4) extrathyroidal sources of thyroid hormone (ectopic thyroid tissue or exogenous hormone). Patients with hyperthyroidism demonstrate a spectrum of hypermetabolic features, including nervousness, palpitations, muscle weakness, increased appetite, diarrhea, heat intolerance, warm skin, weight loss, and perspiration. Affected patients also may have exophthalmos, emotional changes, menstrual changes, and a fine tremor of the hands. In the presence of a clinical history and physical examination consistent with hyperthyroidism, a diagnosis of hyperthyroidism (but not necessarily its cause) can be established by the demonstration of a low TSH level and a high free T₄. In uncommon situations, only the total T₃ level is elevated and the serum free T₄ is normal (T₃ thyrotoxicosis). To determine the etiology of the hyperthyroidism, additional testing is usually necessary. Graves disease, toxic multinodular goiter (TMNG), and toxic adenoma account for the vast majority (>95%) of cases of hyperthyroidism. It should be noted that diffuse or focal enlargement of the thyroid gland, also known as goiter, can be associated with hyperfunction, normal function, and hypofunction of the gland.

Thyroid Storm

Thyroid storm is a relatively uncommon, but life-threatening manifestation of hyperthyroidism caused by excess circulation of thyroid hormones. Symptoms of thyroid storm are similar, but much more severe than traditional hyperthyroidism, including a markedly high fever of 105°F to 106°F, tachycardia, hypertension, and neurological and gastrointestinal abnormalities. Thyroid storm is precipitated by acute illnesses such as sepsis, diabetic ketoacidosis, and preeclampsia, as well as surgical or other diagnostic or therapeutic actions such as radioactive iodine use, anesthesia, excessive thyroid hormone ingestion, or thyroid palpation. Thyroid storm is associated with a high fatality rate if not identified early. The diagnosis is based on the presence of clinical signs and symptoms of severe hyperthyroidism in the context of a precipitating cause. In addition, marked elevations in free and total T₄ are common in thyroid storm. Total T₃ is unreliable in this setting because concomitant nonthyroidal illness (NTI) may cause T₃ to decrease significantly.

Graves Disease

Graves disease is a relatively common hyperthyroid disorder occurring more frequently in women. It has a familial predisposition. It is an autoimmune disease caused by TSH receptor antibodies (TRAbs) that bind to and stimulate TSH receptors resulting in autonomous production of thyroid hormone. While many patients have the classic signs and symptoms of thyrotoxicosis, in elderly patients with Graves disease, apathy, muscle weakness, and cardiovascular abnormalities occur more often than hypermetabolic symptoms.

Laboratory tests show undetectable TSH and increased free T₄. In some cases, the T₃ is elevated and the T₄ is normal. The differential diagnosis includes TMNG, toxic adenoma, painless and subacute thyroiditis, ectopic thyroid tissue, and anxiety states (see below for descriptions). Detection of TRAbs along with the results from radioactive iodine uptake and nuclear thyroid scans are helpful in distinguishing among these possibilities. There is usually increased radioactive iodine uptake in Graves disease. The pattern on imaging is diffuse.

Toxic Multinodular Goiter

The cause of hyperthyroidism in patients with TMNG is an apparent functional autonomy of certain areas within the thyroid gland. The disorder is seen more commonly in elderly patients. The degree of hyperthyroidism is generally less severe than that found in Graves disease. Cardiovascular symptoms are prominent, such as arrhythmias, atrial fibrillation, or congestive heart failure, with weakness and wasting.

Laboratory tests usually show low or undetectable TSH and normal or elevated free T₄ and T₃ concentrations and no evidence of thyroid autoantibodies. Patients with TMNG will have normal to high radioactive iodine uptake, and the thyroid scan shows iodine localized to active nodules.

Toxic Adenoma

Thyroid adenomas that secrete thyroid hormones and cause hyperthyroidism are known as toxic adenomas. Thyroid hormone synthesis by a toxic adenoma is usually independent of TSH regulation, and it results in suppression of TSH secretion. These tumors can usually be distinguished from TMNG and Graves disease by a radioactive iodine uptake study and thyroid scan because there is localized uptake in the adenoma and little or no uptake in surrounding thyroid tissue.

Thyroiditis

Subacute Thyroiditis.

Subacute thyroiditis is produced by a viral infection that alters thyroid function. This disease usually lasts for months, with thyroid function eventually returning to normal. The patient often has an associated upper respiratory infection, fever, and local pain mimicking a sore throat or an earache.

Patients in the early stage of this disease may have hyperthyroidism, with elevated T₄ and T₃ concentrations and a low TSH. Laboratory findings also often include a high erythrocyte sedimentation rate and little to no radioactive iodine uptake. If the disease progresses and the thyroid hormones are depleted, the patient develops hypothyroidism with low T₃ and T₄ concentrations and an elevated TSH.

Postpartum Thyroiditis.

Postpartum thyroid disease is a transient inflammatory process that has an onset of 1 to 6 months postpartum. Although the etiology is unclear, it is thought to be caused by a rebound in the immune system in response to the general state of immunosuppression that occurs during pregnancy. This disease can present as either hyperthyroidism or hypothyroidism. The typical disease course begins with a period of hyperthyroidism with elevated free T₄, reduced TSH, and little to no radioactive iodine uptake for 3 to 6 months. This is followed by a 3- to 6-month period of hypothyroidism associated with reduced concentrations of free T₄ and elevated TSH that completely resolves after 1 year. Approximately 20% of women with postpartum hypothyroidism develop permanent disease, requiring lifelong treatment and monitoring. The presence of anti-TPO antibodies prior to and during pregnancy is associated with an increased risk for postpartum thyroiditis.

Painless Thyroiditis.

Painless thyroiditis may be induced by numerous drugs including lithium, interferon, and in a small portion of patients on amiodarone therapy. Further, some patients with chronic thyroiditis have a transient painless thyrotoxicosis, of unclear etiology.

Typically, free T₄ and T₃ concentrations are elevated with a low TSH. Patients have a markedly depressed radioactive iodine uptake. Painless thyroiditis can be distinguished clinically from subacute thyroiditis because an elevated erythrocyte sedimentation rate and local pain in the region of the thyroid are more consistent with subacute thyroiditis. A definitive diagnosis can be made by microscopic review of cells obtained by aspiration or biopsy. Thyroglobulin measurements can differentiate among patients with chronic thyroiditis from those with thyrotoxicosis caused by surreptitious thyroid hormone intake.

Hypothyroidism Overview and Associated Disorders

Description and Diagnosis

When hypothyroidism occurs during development and in infancy, it results in a condition known as cretinism, which is marked by retardation of physical and intellectual growth. In 95% of cases, hypothyroidism originates in the thyroid gland itself. If a patient has an increased serum TSH and a decreased free T₄—together with appropriate clinical symptoms—a diagnosis of hypothyroidism is confirmed (Table 22–1). In asymptomatic patients, increased TSH, accompanied by a normal free T₄, is known as subclinical hypothyroidism and may be indicative of early stages of primary hypothyroidism. High titers of anti-TPO antibodies suggest Hashimoto thyroiditis (see below) or postpartum thyroid dysfunction in a postpartum woman. While in the United States, autoimmunity is the main cause of hypothyroidism, iodine deficiency is the primary cause worldwide.

Hypothyroidism also may be a result of inadequate stimulation of the thyroid by TSH. This is known as secondary hypothyroidism. A subnormal free T₄ with a decreased or inappropriately normal TSH is suggestive of secondary hypothyroidism from decreased TSH production or production of a biologically inactive form of TSH in the pituitary. It is usually accompanied by other pituitary hormone deficiencies, and it is much less common than primary hypothyroidism.

Clinical pictures of hypothyroidism differ, depending on the age. Congenital hypothyroidism is characterized by low production of thyroid hormones and can result in growth and intellectual delay if untreated. In the United States, all states screen for congenital hypothyroidism by testing for elevated TSH or a combination of elevated TSH and decreased free T₄. Significant changes in thyroid function occur in the neonatal period and throughout childhood. Therefore, TSH and free T₄ concentrations should be assessed using age-specific reference intervals. In particular, T₄ is typically elevated in newborns. In adults, hypothyroidism can have an insidious onset, especially in the elderly. Symptoms are usually nonspecific in the early stage and then progress to more definitive characteristics of hypothyroidism with dry hair, dry skin, periorbital puffiness, dull expression, large tongue, and enlarged heart. If untreated, myxedema coma with respiratory failure may occur. Treatment involves hormone replacement.

Hashimoto Thyroiditis

Hashimoto thyroiditis is a common chronic inflammatory disease of the thyroid that accounts for as many as 90% of all cases of hypothyroidism in areas of iodine sufficiency. Autoimmune factors are thought to be the cause. Hashimoto thyroiditis is often associated with other autoimmune diseases such as Sjögren syndrome and pernicious anemia.

Patients with Hashimoto thyroiditis carry anti-TPO and antithyroglobulin antibodies. Firm thyroid enlargement and goiter is characteristic, but atrophy is also seen. Patients typically have an increased TSH and may have a normal free T₄ and an elevated radioactive iodine uptake in the early stage of the disease. Over time, serum T₄ declines first, followed by a decline in T₃ as hypothyroid symptoms become predominant.

Postablative Hypothyroidism

Postablative hypothyroidism is a relatively common cause of hypothyroidism in adults. Thyroid ablation occurs with total or subtotal thyroidectomy, or following treatment with radioactive iodine for hyperthyroidism.

A history of ablative therapy along with an elevated TSH and a low free T₄ concentration indicates that ablation has produced a hypothyroid state.

Infantile Hypothyroidism

Severe hypothyroidism in infancy is known as cretinism and, as previously noted, is characterized by irreversible mental retardation and growth impairment unless treated promptly. The appearance of symptoms depends on the severity of the disorder. However, even severe hypothyroidism is not usually apparent at birth. Early diagnosis and treatment with thyroid hormone prevents the manifestations of the disease. Elevated TSH and a low T₄ in a newborn or young infant are indicative of infantile hypothyroidism.

Pregnancy-related Thyroid Disease

Normal pregnancy is associated with a number of physiologic changes in thyroid function resulting in differences in “normal” laboratory values for thyroid function tests. The increase in estrogen stimulates hepatic synthesis of TBG, resulting in a net increase in total T₃ and total T₄ by about 1.5-fold. Significant homology exists between hCG, the pregnancy-associated glycoprotein hormone (see Chapter 20), and TSH. Because of this, hCG can directly stimulate the thyroid to produce thyroid hormone. Excess production of thyroid hormone signals a downregulation of TSH secretion. In the first trimester of pregnancy, increasing hCG concentrations are directly mirrored by decreasing TSH concentrations, which return to low normal in the second and third trimesters. Thus, TSH measurements in pregnancy should be considered in the context of gestational age and a reduced upper limit of normal. Many laboratories now report TSH with trimester-specific reference intervals.

Thyroid dysfunction during pregnancy can result in increased risks for the mother and fetus. Hypothyroidism during pregnancy is associated with an increased risk of miscarriage or preterm delivery and impaired neurological development in the fetus. Although controversial, it is recommended to screen all high-risk and symptomatic pregnant women for hypothyroidism by measuring TSH. Hyperthyroidism in pregnancy is associated with an increased risk of spontaneous abortion, preterm delivery, preeclampsia, and thyroid anomalies in the newborn. A subnormal TSH test result should be followed with free T₄ testing. An elevated free T₄ with the presence of autoantibodies confirms the diagnosis of hyperthyroidism in pregnancy. In mothers with confirmed thyroid disease, fetal thyroid function can be evaluated with ultrasound and amniotic fluid testing for TSH, free T₄, and total T₄. Normal reference intervals are instrument-specific for amniotic fluid thyroid function tests.

Euthyroid Sick Syndrome

Description

It is estimated that 40% of emergency department patients have euthyroid sick syndrome (ETS) at presentation. Stress, trauma, and illness can alter thyroid hormone production, transport, and metabolism, and thereby TSH levels, because of disruption of the normal feedback relationship between TSH and T₃ and T₄. This condition with altered thyroid hormone levels and no intrinsic disorder of the thyroid gland is called ETS or NTI, of which there are several variants.

Diagnosis

There is no consensus in the literature regarding the diagnosis and also therapy of ETS. The cause of ETS is different from patient to patient and is dependent on the history and any endocrinologic diagnosis. In moderately ill patients with ETS, serum T₄ concentrations are within the reference range, while serum T₃ is decreased and rT₃ is increased. Serum TSH concentrations are typically normal to low (except for a transient increase that may occur during recovery). However, it is not recommended to measure TSH in hospitalized patients unless there is a strong suspicion for thyroid disease. In seriously ill patients, ETS presents with low-normal T₄ and significantly reduced total T₃ concentrations. Serum rT₃ is increased because of slow thyroid hormone clearance and greater than normal conversion of T₄ to rT₃ rather than to T₃. An elevated rT₃ in the appropriate clinical setting, with appropriately suggestive laboratory test results, points to ETS syndrome.

Thyroid Tumors

Description

Masses or “nodules” in the thyroid may be associated with normal function, hyperfunction, or hypofunction, and for that reason, they are considered apart from hyperthyroidism and hypothyroidism. In fact, some studies suggest that 25% to 40% of the population have thyroid nodules. Most solitary masses detected with physical examination are the dominant nodule in a multinodular goiter, a cyst, or an asymmetric enlargement of the gland. Benign thyroid adenomas account for most of the neoplastic nodules. The initial evaluation for a thyroid nodule is to measure TSH and perform a thyroid ultrasound. If the TSH is suppressed, a nuclear scan can be performed. Hyperfunctioning nodules appear as “hot nodules” by nuclear scan because they take up radioactive iodine while uptake is suppressed in the remainder of the gland. A nonfunctional (cold) nodule carries an increased risk of malignancy and should be followed with ultrasound-guided FNA or biopsy. The morphologic variants of thyroid cancer, diagnosed with histopathologic review of a biopsy specimen or aspirate (in order of frequency), are papillary carcinoma, follicular carcinoma, medullary thyroid carcinoma, and anaplastic carcinoma.

Diagnosis

The diagnosis is established by histopathologic review of a specimen obtained with FNA or biopsy. The accuracy of the diagnosis is increased with the use of guided ultrasound examination for sample collection. Treatment and risk of recurrence of thyroid neoplasms are assessed by periodic measurement of tumor markers, including thyroglobulin for papillary and follicular carcinomas, and calcitonin for MTC.

Physiology and Biochemistry

The adrenal cortex secretes many steroid hormones that have a wide variety of physiologic effects. The hormones can be grouped into 3 major categories: glucocorticoids, mineralocorticoids, and sex steroids that include androgens, progestogens, and estrogens. The glucocorticoids and mineralocorticoids are collectively known as corticosteroids. Steroid hormones are synthesized from cholesterol in the adrenal glands and in the gonads. They are transported in the blood bound to carrier proteins, such as albumin and hormone-binding globulins, or as free hormone. Steroids may be modified with glucuronate or sulfate to increase their water solubility and permit excretion via the kidneys or the gastrointestinal tract. The percentage of steroid hormone that is bound to protein varies with the hormone affinity for carrier proteins and ranges from 60% to nearly 100%. Quantitatively, the glucocorticoids and mineralocorticoids are the most important group of hormones produced by the adrenal cortex. The major corticosteroids are cortisol (a glucocorticoid) and aldosterone (a mineralocorticoid). The synthesis and metabolism of the steroid hormones are illustrated in Figure 22–4. The liver is the main site for conjugation of steroid hormones, and the kidney excretes approximately 90% of the conjugated steroids. Glucocorticoids alter carbohydrate metabolism by increasing gluconeogenesis and decreasing glucose utilization. Additional effects include the inhibition of amino acid uptake and protein synthesis in peripheral tissues. Mineralocorticoids promote sodium conservation and potassium loss and thereby considerably influence the retention or loss of fluid. Among the naturally occurring mineralocorticoids, aldosterone has the highest mineralocorticoid activity followed by deoxycorticosterone and corticosterone.

Secretion of adrenal glucocorticoids and adrenal androgens is regulated by corticotropin (ACTH) that is secreted by the pituitary gland (Figure 22–5). ACTH also plays a minor role in aldosterone and mineralocorticoid production. Their synthesis is primarily controlled by a different pathway known as the renin–angiotensin system. The hypothalamic–pituitary–adrenal (HPA) axis begins with the episodic release of corticotropin-releasing hormone (CRH) from the hypothalamus. CRH stimulates the episodic release of ACTH from the pituitary. ACTH then stimulates the adrenal cortex to produce cortisol in a diurnal or circadian manner. Cortisol concentrations are highest in the early morning between 4 and 8 AM and about 25% lower in the late evening. Physical and mental stress can elevate cortisol concentrations and blunt the circadian rhythm. ACTH release is under negative feedback control from the cortisol fraction not bound to proteins. Two adrenal sex steroids dehydroepiandrosterone (DHEA) and androstenedione are also stimulated by ACTH as well as other hormones including insulin-like growth factor-1 and gonadal steroids. Adrenal androgen production reaches a peak in the second decade, with a rise during late childhood. It gradually decreases and reaches a low level in the elderly.

As shown in Figure 22–6, aldosterone secretion is primarily controlled by the renin–angiotensin system. Renin is an enzyme synthesized and stored in cells of the juxtaglomerular afferent arterioles of the renal glomeruli. The circulating renin hydrolyzes angiotensinogen to produce angiotensin I, which is rapidly converted to angiotensin II by angiotensin-converting enzyme. Angiotensin II then stimulates the cells of the adrenal cortex to produce aldosterone. Angiotensin II is also a potent vasoconstrictor. The primary stimuli for renin release are decreased perfusion of the kidney and a negative sodium balance.

Laboratory Tests

The functional status of the adrenal cortex can be evaluated by measuring the circulating concentrations of components of the HPA axis and the renin–angiotensin system. In addition to measurement of the plasma, serum, or urinary concentrations of these compounds, dynamic stimulation and suppression tests are valuable in identifying certain abnormalities.

Cortisol

Because the secretion of cortisol is diurnal and pulsatile, a single, random serum cortisol measurement is not useful in the diagnosis of adrenal dysfunction. However, a decreased early morning serum cortisol measurement may suggest adrenal insufficiency. The 24-hour urinary excretion of cortisol, an index of plasma-free cortisol during that 24-hour time frame, is a reliable gauge of excess cortisol secretion by the adrenal cortex. This is because taking an average urine cortisol concentration over 24 hours eliminates the need to account for diurnal variation as in a single serum collection. Urinary free cortisol (UFC) measurements should not be used in patients with renal impairment. Late-night salivary cortisol is also a measure of free cortisol since the binding protein does not cross into saliva. Salivary cortisol concentrations correlate well with serum concentrations and are not influenced by changes in cortisol-binding protein concentrations. Measurement of late-night salivary cortisol is an acceptable screening method for hypercortisolism.

Low-dose Dexamethasone Suppression Tests

Dexamethasone is a synthetic glucocorticoid more potent than cortisol that, when given orally or IV, suppresses ACTH and CRH secretion. It can be administered as 1 mg at midnight in an overnight dexamethasone suppression test, or in a 2-day low-dose dexamethasone suppression test (LDDST) by giving the patient 0.5 mg every 6 hours for 8 doses. If the dexamethasone is effective, it will suppress ACTH secretion and, thereby, suppress cortisol production. Patients with Cushing disease normally do not show suppression of cortisol synthesis after either dexamethasone administration for the LDDST or dexamethasone in the overnight suppression test.

ACTH

In patients with an abnormal dexamethasone suppression test, ACTH measurements are important to determine whether Cushing syndrome is ACTH-dependent. The optimum time of day for determination of plasma ACTH concentration in patients with suspected Cushing syndrome is between midnight and 2 AM when the plasma-circulating concentrations of ACTH and cortisol are at their lowest point. At this time, the ability to detect an abnormality in ACTH secretion is the greatest. In patients with suspected adrenal insufficiency, ACTH should be measured in the morning during its peak. Suppressed morning ACTH may indicate excess adrenal cortisol secretion.

ACTH Stimulation Test

The ACTH stimulation test is the most useful test in diagnosis of adrenal insufficiency. It assesses the secretory response of the adrenal cortex to an ACTH-like stimulus. A 250-mg dose of an ACTH analog, Cortrosyn or cosyntropin, is administered IV or IM and serum cortisol is measured at 0, 30, and 60 minutes after injection. Normally, ACTH analogs will stimulate production of cortisol to concentrations >20 mg/dL. Lack of rise in cortisol after ACTH stimulation may indicate primary and secondary adrenal insufficiency.

CRH Stimulation Test

CRH can be administered intravenously to determine the response of the pituitary to stimulation by hypothalamic hormone. The CRH stimulation test can be used to determine the source of adrenal insufficiency in patients with an abnormal ACTH stimulation test. Synthetic CRH is administered IV, and then ACTH and cortisol are measured at 0, 30, 60, 90, and 120 minutes after injection. Normal patients will respond to CRH by stimulating ACTH and cortisol production. Patients with primary adrenal insufficiency will have elevated ACTH, but no cortisol production, while patients with secondary disease will have both low ACTH and cortisol.

Aldosterone

The most important test to establish a diagnosis of hyperaldosteronism or hypoaldosteronism is plasma aldosterone. Aldosterone can be measured in the plasma as the unmodified hormone, and in the urine as the 18-glucuronide conjugated metabolite of aldosterone. Screening for primary aldosteronism should include the determination of the ratio of plasma aldosterone concentration (PAC)/plasma renin activity (PRA), that is, the aldosterone to renin ratio (ARR). Aldosterone and renin concentrations are affected by numerous conditions and medication. Therefore, it is recommended that testing be performed under the following conditions: patients with normal potassium concentrations, patients with normal salt intake, patients removed from drugs that affect the ARR, and midmorning collection in patients who have been ambulatory for at least 2 hours.

Renin

PRA is assayed by measuring its ability to convert angiotensinogen to angiotensin I, which is then quantitated by an immunoassay. When primary aldosteronism is present, often because of either resistant or severe hypertension, the ARR will be elevated. Renin mass can also be assessed directly by immunoassay, to determine the direct renin concentration (DRC). Although the ARR can be calculated using DRC or PRA, fewer studies have investigated the utility of DRC. Renin specimens should not be stored refrigerated or on ice prior to analysis as cold temperatures promote secretion of prorenin, which falsely elevates results. See the section “Aldosterone” for recommended testing conditions.

Steroid Measurements to Identify Enzyme Deficiencies in Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia (CAH) represents a spectrum of diseases resulting from enzyme deficiencies that impair normal hormone synthesis in the adrenal cortex. The decrease in cortisol production in most forms of CAH leads to an overproduction and shunting into the androgen synthesis pathway. These disorders are described in the section “Alterations in the Synthesis of Glucocorticoids, Mineralocorticoids, and Sex Steroids: Congenital Adrenal Hyperplasia.” The assays used to identify the specific enzyme deficiencies include measurement of 17-hydroxyprogesterone (17-OHP), either unstimulated or after ACTH stimulation, 17-hydroxypregnenolone after ACTH stimulation, deoxycorticosterone, 11-deoxycortisol, and several androgens (androstenedione, DHEA, and testosterone). DNA-based tests also can be used to identify certain gene mutations that result in enzyme deficiencies found in CAH.

Hyperfunction Involving Glucocorticoids With or Without Mineralocorticoids: Cushing Syndrome

Description

Cushing syndrome is a disorder of excess cortisol production, which is more commonly ACTH-dependent than ACTH-independent. The early clinical features of Cushing syndrome are hypertension and weight gain. Truncal obesity with a round face, often known as a “moon facies,” and an accumulation of fat in the posterior neck and regions of the back close to the neck, known as a “buffalo hump,” appear with progression of the disease. Decreased muscle mass and proximal limb weakness occur from atrophy of muscle fibers induced by the high level of cortisol, which inhibits protein synthesis and uptake of glucose by the cells. Patients with Cushing syndrome often have elevated blood glucose levels with glucosuria. Other clinical signs and symptoms include striae of the skin, osteoporosis, hirsutism, menstrual abnormalities in women, and mental status changes involving mood swings with depression.

Cushing syndrome is separated into 3 main disease entities, with increased synthesis and loss of circadian rhythm resulting in excess cortisol production by the adrenal cortex as the common theme. Independent of the 3 naturally occurring forms of Cushing syndrome, the administration of glucocorticoids as a medication is a common cause of Cushing syndrome. This should be evident from the medication history. Endogenous forms of Cushing syndrome are described as follows:

• Cushing disease is the most common form of Cushing syndrome and is 4- to 6-fold more prevalent in women. It is caused most often by small ACTH-secreting tumors in the pituitary (<1 cm in size) known as microadenomas. These adenomas can be detected with various radiographic techniques after appropriate hormone tests suggest a pituitary etiology. On rare occasions, the tumors are large and present as macroadenomas.

• Adrenal Cushing syndrome—This form of Cushing syndrome is most commonly associated with a benign or malignant tumor in the adrenal cortex. Adrenal adenomas synthesize cortisol efficiently, but adrenal carcinomas often synthesize cortisol inefficiently and overproduce sex steroids, resulting in virilization.

• Cushing syndrome from ectopic ACTH production—Small cell lung carcinoma and bronchial carcinoid patients account for most of the Cushing syndrome cases in this category. This form of Cushing syndrome is more common in men because of the higher incidence of lung cancer in men. In the absence of signs and symptoms specifically associated with the lung carcinoma or carcinoid, these patients are clinically indistinguishable from those with pituitary Cushing disease. Because the syndrome often appears in patients with significant clinical manifestations of cancer, the symptoms from ectopic ACTH production often go unrecognized. Another very rare cause of Cushing syndrome with an ectopic focus and high serum ACTH level is a tumor, most frequently a bronchial carcinoid tumor, which secretes CRH instead of ACTH.

Diagnosis

The diagnosis of Cushing syndrome, or Cushing disease, discussed below collectively as Cushing syndrome, requires evidence of increased cortisol production and loss of suppression of cortisol synthesis or loss of cortisol diurnal variation. Screening for Cushing syndrome is difficult because 1) its prevalence is low; 2) several common conditions can produce biochemical and clinical signs of hypercortisolism in the absence of Cushing syndrome; and 3) the screening tests have a high rate of false-positive results for Cushing syndrome. For this reason it is recommended that a careful history be taken to exclude exogenous causes of hypercortisolism and that only patients with high clinical suspicion, such as those with unusual symptoms, for example, osteoporosis and less than 40 years old, children with a decrease in height and increase in weight percentiles, severe symptoms, or adrenal tumors visualized by imaging, be screened for Cushing syndrome.

The strategy for the diagnosis of Cushing syndrome, and the subsequent identification of 1 of the 3 forms of Cushing syndrome, involves tests to confirm endogenous hypercortisolism and then to determine whether the disease is ACTH-dependent. ACTH concentrations are low in patients with adrenal tumors, but normal or elevated with pituitary or ectopic ACTH-producing tumors. Table 22–2 summarizes the laboratory evaluation for Cushing syndrome.

The following steps to diagnose and differentiate Cushing syndrome can be made using first-line and then second-line diagnostic tests, respectively.

Diagnostic Evaluation

It is recommended that at least one of the following testing strategies be performed to diagnose patients with Cushing syndrome:

1. The 24-hour UFC or late-night salivary cortisol is a sensitive screening test for patients with clinical signs and symptoms of Cushing syndrome. Because of the variability of the cortisol concentrations, the UFC or salivary cortisol should be elevated on at least 2 separate occasions in order to proceed with the diagnostic evaluation for Cushing syndrome. If the UFC or salivary cortisol is normal in a high-risk patient, an endocrinologist should be consulted for further studies. A low-risk patient with normal results should be rescreened in 6 months if signs or symptoms persist or worsen.

2. In unhealthy, but not critically ill, patients, and/or those with abnormal renal function, or disrupted sleep patterns, physicians should use one of the LDDSTs to screen for Cushing syndrome. If cortisol production is suppressed by a low dose of dexamethasone in an overnight or 48-hour test, Cushing syndrome is ruled out. If the clinical suspicion for Cushing syndrome is still high in the presence of a nonsuggestive result for Cushing syndrome, further evaluation by an endocrinologist may be useful. A lack of cortisol suppression after a low dose of dexamethasone is suggestive of Cushing syndrome. Patients taking oral estrogens should not be screened with the LDDST, as estrogens stimulate the liver to synthesize cortisol-binding globulin (CBG) and elevate serum cortisol concentrations, causing false-positive screening results. Further, critically ill patients synthesize less CGB and albumin, leading to lower serum cortisol concentrations, and possibly false-negative screening results.

3. Other clinical conditions that cause hypercortisolism should be ruled out. These include alcoholism, severe obesity, pregnancy, depression, diabetes mellitus, and glucocorticoid resistance.

Tests to Identify Etiology

Once Cushing syndrome is confirmed, a series of tests can then be performed to locate the cause of the hypercortisolism.

1. Plasma ACTH testing should be performed to determine the etiology of the hypercortisolism. If ACTH is suppressed, an adrenal source is suspected. If ACTH is high or inappropriately normal, the patient may have a pituitary adenoma.

2. If an ACTH-secreting pituitary adenoma is suspected, MRI of the pituitary gland to identify a mass or petrosal sinus blood sampling to collect samples coming directly out of the pituitary after CRH stimulation is used as a confirmation test. A ratio of petrosal ACTH to serum ACTH above suggested cutoff values before and after CRH administration is consistent with a pituitary cause.

3. If the ACTH is suppressed, a CT of the adrenal glands is likely to be informative to identify an adrenal tumor and indicate an adrenal cause.

4. If an ectopic ACTH-secreting tumor is suspected, this can be challenging to locate, but imaging studies are often valuable. Chest, pancreas, colon, and gall bladder carcinomas have been shown to be sources of cortisol secretion.

“Pseudo-Cushing syndrome,” which can be produced by alcohol abuse and other disorders, mimics both the clinical and biochemical features of the true syndrome.

Hypofunction Involving Glucocorticoids With or Without Mineralocorticoids: Adrenal Insufficiency

Description

Adrenal insufficiency can be either primary, from destruction of the adrenal cortex by a local disease process or a systemic disorder, or secondary from pituitary or hypothalamic disease that reduces stimulation of the adrenal gland. The most common causes of primary adrenal insufficiency, in which all classes of adrenal cortical steroids are deficient, are autoimmune adrenalitis and tuberculosis in endemic regions. In this particular disorder, the adrenal medulla and its catecholamine synthesis are spared. In other primary adrenal insufficiency disorders in which the adrenal gland is damaged, the adrenal medulla may be damaged along with the adrenal cortex. In secondary adrenal insufficiency, in which there is deficient stimulation of the adrenal gland because of pituitary or hypothalamic abnormalities, the adrenal medulla is not affected and aldosterone deficiency is not usually present. Aldosterone secretion is more dependent on angiotensin II stimulation of the adrenal cortex than on stimulation of the adrenal cortex by ACTH (as discussed below).

• Primary adrenal insufficiency—There are many causes of primary adrenal insufficiency. Dysfunction in 1 or more sites in the HPA axis is the major cause of the primary adrenal insufficiency. Chronic primary adrenal insufficiency, also known as Addison disease, occurs mostly in adults. The most common causes are autoimmune disease (Western world) and tuberculous adrenalitis (worldwide). Other causes of primary adrenal insufficiency include fungal or viral infections (ie, histoplasmosis or HIV), and anatomic destruction of the adrenal glands through surgery, hemorrhage, or metastatic carcinomas. Primary adrenal insufficiency is characterized by hyperpigmentation of the skin and mucous membranes. The lack of negative feedback from adrenal cortisol leads to increased ACTH production by the pituitary. A degradation product of ACTH and its prohormone pro-opiomelanocortin is melanocyte-stimulating hormone (MSH). MSH stimulates melanin production and induces melanocyte hyperpigmentation. Hyperkalemia and hypotension may be present if there is deficient aldosterone (see the sections “Primary Hypoaldosteronism” and “Secondary Hypoaldosteronism”). Primary adrenal insufficiency usually has a gradual onset but may occur abruptly with a stressor such as critical illness or surgery. The rapid onset form of adrenal insufficiency is often the result of adrenal hemorrhage or thrombosis that impairs blood supply to the gland.

  Acute primary adrenocortical failure (also called Addisonian crisis) can be triggered by a severe infection, sepsis, or abrupt withdrawal of steroids. It is a life-threatening emergency, characterized by abnormal electrolytes (critically high potassium and low sodium), hypotension, and hypoglycemia. Sudden death can occur if it is not treated promptly.

• Secondary adrenal insufficiency—A deficiency of ACTH secretion from any cause can lead to adrenal insufficiency. Long-term glucocorticoid therapy can result in prolonged suppression of CRH from the hypothalamus and ACTH from the pituitary and transient adrenal insufficiency. Hypopituitarism as a result of postpartum hemorrhage (Sheehan syndrome), radiation, surgery, or injury may result in decreased ACTH production leading to reduced glucocorticoid synthesis.

Diagnosis

The management of adrenal insufficiency first requires determination of the disease source (ie, primary or secondary), followed by an identification of the specific cause for the adrenal insufficiency. The tests that are useful in the diagnosis of primary and secondary adrenal insufficiency are shown in Table 22–3. Adrenal insufficiency may involve a deficiency of glucocorticoids or both glucocorticoids and mineralocorticoids. Depending on the extent of adrenal cortex damage, patients may have decreased serum cortisol and/or decreased plasma aldosterone levels. The ACTH stimulation test is the most specific test to confirm a diagnosis of adrenal insufficiency. Patients with primary adrenal insufficiency do not usually show cortisol secretion following ACTH stimulation because the defect is within the adrenal gland. Patients with mild or recent onset of secondary adrenal insufficiency (and a still viable adrenal cortex) respond to the ACTH because the defect is not within the adrenal gland. A chronic lack of stimulation of the adrenal cortex by ACTH in secondary adrenal insufficiency can result in cortical atrophy and limited cortisol production following ACTH stimulation. The CRH stimulation test can distinguish between secondary adrenal insufficiency caused by ACTH deficiency and that caused by CRH deficiency. Plasma ACTH and serum cortisol are measured after administration of CRH; increases are observed in hypothalamic disorders, but not in pituitary disorders. A serologic test for antiadrenal antibodies is useful to determine if autoimmune adrenalitis is the cause of primary adrenal insufficiency.

Hyperfunction and Hypofunction Involving Mineralocorticoids: Hyperaldosteronism and Hypoaldosteronism

Description and Diagnosis

Aldosterone is a mineralocorticoid produced in the adrenal glands. It is largely responsible for regulating sodium retention and water resorption, and thereby control of blood volume. It also promotes the excretion of potassium into the urine. Aldosterone concentration in the blood is regulated by the renin–angiotensin aldosterone (RAA) system (Figure 22–6). In response to decreased blood volume, the juxtaglomerular apparatus of the kidney secretes renin, which converts angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme in the lungs. Angiotensin II is a potent vasoconstrictor and also stimulates the adrenal glands to secrete aldosterone, which then acts to increase blood volume by promoting sodium retention in exchange for potassium that is lost into the urine.

Aldosterone concentrations in disease may be high (hyperaldosteronism) or low (hypoaldosteronism). An abnormal (high or low) PAC may be the result of a defect originating inside (primary disorder) or outside (secondary disorder) of the adrenal gland (Table 22–4).

Primary Hyperaldosteronism

In this disorder, there is excess secretion of aldosterone as a result of an abnormality within the adrenal gland. Most often, hyperaldosteronism is caused by bilateral hyperplasia of the adrenal glands or by an aldosterone-secreting adrenal adenoma, resulting in a disorder known as Conn syndrome. Less often, primary hyperaldosteronism is a result of primary (unilateral) adrenal hyperplasia or a cancerous tumor. In primary hyperaldosteronism, the PRA is low because aldosterone stimulates high sodium concentrations and increased blood volume triggers downregulation of renin secretion. The ratio of the PAC/PRA is widely used as a screening test for primary aldosteronism in hypertensive patients. A ratio of greater than 30 when PAC is measured in conventional units (ng/dL) and 750 when PAC is in SI units (pmol/L) is the most commonly used cutoff to identify patients with this disorder. Because of the effect of numerous drugs, diet, and comorbidities on PAC and PRA concentrations, it is recommended that along with an elevated ratio, patients also have PAC concentrations above 15 ng/dL. Some laboratories use DRC as an indirect measure of renin activity. A few studies have published recommended diagnostic cutoffs for the PAC/DRC ratios for primary hyperaldosteronism, but they have not yet been universally accepted.

Secondary Hyperaldosteronism

In this disorder, there is excess secretion of aldosterone as a result of an abnormality outside the adrenal gland. It is much more common than primary hyperaldosteronism. Decreased renal perfusion is the most common cause of secondary hyperaldosteronism. The decreased blood flow into the kidney results in an elevation of the PRA. The elevation in plasma renin level (as shown in Figure 22–6) produces the increase in aldosterone. Congestive heart failure, nephrotic syndrome, cirrhosis of the liver, and other hypoproteinemic conditions in which there is chronic depletion of plasma volume can produce an elevation in plasma aldosterone.

The clinical and laboratory features common to both primary and secondary hyperaldosteronism usually include hypertension associated with hypervolemia and low or low-normal concentrations of serum potassium. The serum sodium also may be slightly elevated. Additional clinical features include nocturnal polyuria, polydipsia, and weakness from the low potassium concentrations.

Primary Hypoaldosteronism

This disorder is much less common than primary hyperaldosteronism. Primary hypoaldosteronism is most often a result of destruction of the adrenal gland from various causes (as noted in the section “Hypofunction Involving Glucocorticoids With or Without Mineralocorticoids: Adrenal Insufficiency”), including autoimmune adrenalitis, adrenal infection by tuberculosis, metastatic tumors to the adrenal, adrenalectomy, CAH associated with low aldosterone production (see the section “Alterations in the Synthesis of Glucocorticoids, Mineralocorticoids, and Sex Steroids: Congenital Adrenal Hyperplasia”), and hemorrhage into the adrenal gland. There is an additional disorder associated with primary hypoaldosteronism, known as pseudohypoaldosteronism, in which the tissues are resistant to the action of aldosterone. Low blood volume and/or sodium due to lack of aldosterone action upregulates renin synthesis, which subsequently upregulates aldosterone. Therefore, these patients have significantly elevated PAC and PRA.

Secondary Hypoaldosteronism

In this disorder, aldosterone hyposecretion results from factors originating outside the adrenal gland. One cause is a deficiency of ACTH production in the pituitary, often accompanied by deficiencies of other pituitary hormones. As noted earlier, the adrenal cortex can become atrophied as a result of a chronic lack of stimulation by ACTH, decreasing aldosterone as well as cortisol production. Another cause is long-term glucocorticoid administration. Long-term glucocorticoid-induced ACTH suppression leads to adrenal atrophy and reduced aldosterone synthesis. Secondary hypoaldosteronism can also occur as a result of deficient renin production due to renal damage or drugs and from inhibition of angiotensin-converting enzyme by drugs. Clinical and laboratory features common to primary and secondary hypoaldosteronism include hypotension, which may be orthostatic, and high serum potassium levels. Slightly low serum sodium values also may be present. The clinical signs and symptoms vary significantly and depend on the specific defect leading to the hypoaldosteronism.

Alterations in the Synthesis of Glucocorticoids, Mineralocorticoids, and Sex Steroids: Congenital Adrenal Hyperplasia

Description and Diagnosis

CAH is caused by any 1 of a group of enzyme deficiencies in the biosynthetic pathways for cortisol and aldosterone. Because cortisol production is decreased, and cortisol provides the inhibitory feedback to the pituitary for ACTH secretion, there is an increase in ACTH and excess stimulation of the adrenal glands (see Figures 22–5 and 22–6 for the regulation of cortisol and aldosterone production). This results in greater flux through pathways around an existing enzymatic defect, producing elevations in adrenal hormones whose synthesis is not affected by the enzyme deficiency. Most of the known enzyme deficiencies in the synthetic pathways for aldosterone and cortisol result in an elevation in sex steroid synthesis, which has a virilizing effect on the patient. The most common of the enzymatic defects is a deficiency of 21-hydroxylase. This deficiency accounts for 90% to 95% of the cases of CAH. The clinical manifestations for 4 of the enzyme deficiencies producing CAH are noted below. Figure 22–4 shows the intermediate compounds in the synthesis of aldosterone, cortisol, and androgens in the adrenal gland and the enzymes in the pathway, some of which may be deficient. Table 22–5 presents the laboratory evaluation for CAH.

• 21-Hydroxylase deficiency—Deficiency of this enzyme is the most common form of CAH. In female infants, a 21-hydroxylase deficiency usually results in hypertrophy of the clitoris and pseudohermaphroditism as a result of disruption in the synthesis pathways for cortisol and aldosterone and shunting of intermediates down the androgen synthesis pathway. In postpubertal females, it results in amenorrhea, infertility, and hirsutism. In males, the virilization results in enlargement of the external genitalia and precocious puberty. As seen in Figure 22–4, 21-hydroxylase deficiency will result in a reduced or no synthesis of both cortisol and aldosterone and accumulation of the 17-OHP intermediate. As a result, if infants with this deficiency are not treated with corticosteroids, they can quickly develop life-threatening hyperkalemia, hyponatremia, and hypotension. Because of the severe clinical manifestations, it is recommended that all newborns be screened for 21-hydroxylase deficiency by measurement of 17-OHP. Positive newborn screening results should be followed with sensitive and specific confirmatory testing.

• 11-Beta-hydroxylase deficiency—This is the second most common enzyme deficiency responsible for CAH. In infancy, the clinical and laboratory features of patients with this abnormality are largely similar to those found in patients with 21-hydroxylase deficiency. However, deficiency of this enzyme causes accumulation of 11-deoxycorticosterone, a potent mineralocorticoid. Thus, these patients develop mineralocorticoid-induced hypertension and hypokalemia.

• 17-Alpha-hydroxylase deficiency—This deficiency is rare and accounts for approximately 1% of all CAH cases. In this deficiency, there is no inhibition of aldosterone synthesis, but there is a block in the synthesis of both cortisol and sex steroids. The elevation of aldosterone results in hyperaldosteronism that produces hypertension and hypokalemia. In females, the androgen deficiency results in a lack of development of secondary sex characteristics because the androgens are biochemical precursors of estrogens. In males, pseudohermaphroditism appears.

• 3-Beta-hydroxysteroid dehydrogenase deficiency—This is another rare CAH disorder. This enzymatic deficiency results in a metabolic block in the production of aldosterone and cortisol, with no inhibition of the synthesis of DHEA and other androgens. In its severe form, this enzyme deficiency manifests as early masculinization in males and amenorrhea and pseudohermaphroditism in females, as well as life-threatening hyperkalemia, hyponatremia, and hypotension.

Physiology and Biochemistry

The main sites of production of the catecholamines are the brain, the chromaffin cells of the adrenal medulla, and the sympathetic neurons. The catecholamines include dopamine, epinephrine, and norepinephrine as the most potent of the endogenously produced compounds. Of these, in the adrenal medulla, epinephrine production is quantitatively the greatest. The catecholamines have a wide variety of biological effects. They have a marked impact on the vascular system, and are important in blood pressure regulation. Epinephrine influences many metabolic pathways, especially carbohydrate metabolism. In some tissues, epinephrine and norepinephrine produce opposite effects. Alpha-adrenergic receptors on cells interact effectively with norepinephrine and moderately with epinephrine, while beta-adrenergic receptors respond primarily to epinephrine and norepinephrine.

Catecholamine synthesis and metabolism in the adrenal medulla is illustrated in Figure 22–7. The pathway begins when the amino acid tyrosine is metabolized to a catecholamine, dihydroxyphenylalanine (DOPA). DOPA is then converted to dopamine, which is transformed to norepinephrine, which is subsequently converted to epinephrine. Because of their great potency, the catecholamines must be rapidly inactivated through reuptake into storage granules, conversion to metabolites, or excretion. Unlike the steroid hormones, catecholamines are not bound to proteins as they circulate. In plasma, they have a very short half-life of approximately 2 minutes. Urine catecholamines, on the other hand, represent a pool of catecholamines delivered into urine in the preceding hours. There are a number of degradative products of epinephrine and norepinephrine. The compounds noted in Figure 22–7—metanephrine, normetanephrine, and vanillylmandelic acid—are the ones that are measured in clinical assays to assess catecholamine production and degradation.

Laboratory Tests

Epinephrine and Norepinephrine

Total or fractionated (epinephrine or norepinephrine) catecholamines can be measured in plasma or 24-hour urine samples. The plasma concentration reflects the rate of synthesis and release of catecholamines by the adrenal medulla and their half-life in the circulation. Catecholamines are secreted into the urine as free hormones. Urinary catecholamines are extremely unstable and should be acidified during or right after collection.

Metanephrines (Metanephrine and Normetanephrine)

The preferred screening test for adrenal medullary neuroendocrine tumors is detection of free metanephrines and normetanephrines in plasma. Both metanephrine and normetanephrine undergo conjugation with sulfate or glucuronide. The metanephrines can also be measured in a 24-hour urine specimen. Urine measurements are helpful in cases where plasma metanephrines are marginally elevated. Metanephrines are also unstable in urine and should be acidified during or right after collection.

Vanillylmandelic Acid

This compound is the major metabolite of both metanephrine and normetanephrine. It is measured in the urine and, although it is indicative of catecholamine synthesis and metabolism, it is inferior to urinary metanephrine quantitation for this purpose.

Pheochromocytoma

Description

A pheochromocytoma, which may be benign or malignant, is a chromaffin cell tumor of the adrenal medulla or autonomic nervous system that secretes catecholamines. On this basis, it is a cause of hypertension. However, it is a rare cause of hypertension with approximately 5 pheochromocytomas per 100,000 hypertensive cases. Other catecholamine-secreting chromaffin cell tumors are paragangliomas and neuroblastomas. It is essential that a pheochromocytoma be rapidly and accurately identified in patients with hypertension because surgical resection of the tumor, with elimination of the hypertension and its complications, is successful in at least 90% of cases, and the disease may be otherwise fatal. The diagnosis is made most often in patients between the ages of 30 and 60 years. The clinical features of a patient with pheochromocytoma include, most importantly, the presence of sustained or paroxysmal hypertension. The attacks of hypertension occur abruptly and subside slowly, with a total duration of less than 1 hour in approximately 80% of patients. They may be precipitated by palpation of the tumor, postural changes, exertion, anxiety, trauma, pain, intake of foods or beverages containing tyramine (such as certain cheeses, beer, and wine), and the ingestion of certain medications. Headaches are common in patients with pheochromocytoma, and they are usually severe. Generalized sweating and palpitations with tachycardia occur frequently. Other common signs and symptoms are anxiety, chest pain, nausea, fatigue, and weight loss. Of all pheochromocytomas, approximately 25% to 30% are familial and coexist with a form of multiple endocrine neoplasia (MEN), von Hippel Lindau, neurofibromatosis, or succinate dehydrogenase (SDH) mutation (see later section); 10% of inherited pheochromocytomas are malignant; 10% are extra-adrenal in location and are called paragangliomas (and therefore 90% are in the adrenal); and 10% are bilateral and most of these are patients with MEN 2A.

Diagnosis

As noted before, the most biologically significant catecholamines synthesized by a pheochromocytoma are epinephrine and norepinephrine. These compounds are metabolized into metanephrine and normetanephrine, respectively (Figure 22–7), and both of these compounds can be metabolized to vanillylmandelic acid. Measurement of fractionated, free plasma metanephrines is the preferred screening test to rule out pheochromocytoma. This test measures plasma levels of free metanephrine and free normetanephrine as separate compounds. All patients with clinical symptoms and elevated plasma free metanephrines should undergo localization studies with an adrenal CT scan or MIBG (an imaging study involving the use of the radioisotope MIBG). Follow-up testing for patients with suspected pheochromocytoma and a borderline positive plasma metanephrine screening test include measurement of metanephrines in a 24-hour urine sample and then fractionated catecholamines in plasma or urine. The diagnosis of pheochromocytoma is based on the detection of increased concentrations of urinary or plasma metanephrines, and possibly plasma or urinary catecholamines, in the appropriate clinical setting of sustained or paroxysmal hypertension. Adrenal CT or other radiographic techniques can be used to localize a pheochromocytoma. Laboratory tests used for diagnosis of pheochromocytoma are described in Table 22–6.

This section is focused on parathyroid hormone (PTH) and disorders associated with high or low concentrations of this hormone. Most parathyroid disorders alter calcium metabolism, and thereby have an effect on bone. However, there are also a number of disorders associated with hypercalcemia or hypocalcemia, or alterations in bone density, in which a change in the PTH level is not a major factor. Therefore, in addition to hyperparathyroidism and hypoparathyroidism, this chapter also briefly describes a few selected disorders associated with hypercalcemia, hypocalcemia, or altered bone density in which PTH does not play a major role.

Physiology and Biochemistry

PTH is a polypeptide secreted from the parathyroid glands. The primary function of PTH is the regulation of the concentration of ionized calcium in extracellular fluids. An increase in secretion of PTH produces a rise in serum ionized calcium and a decrease in the serum phosphorus concentration. A normal or an elevated blood calcium provides negative feedback to the parathyroid gland to reduce the secretion of PTH (see Figure 22–8).

The resorption of bone induced by PTH is mediated by increased activity of osteoclasts. PTH can also promote an increase in the renal tubular reabsorption of calcium.

Vitamin D is an intermediary in the action of PTH to elevate the serum calcium level. It is a fat-soluble hormone required for calcium absorption in the gut, bone metabolism, and development of cells in the immune system. Vitamin D also influences phosphorus metabolism. Vitamin D₂ is known as ergocalciferol, and vitamin D₃ is known as cholecalciferol.

Food can be fortified with either vitamin D₂ or D₃, both of which can be used as vitamin D supplements. Cholecalciferol is ingested in the diet, and it is also synthesized in the skin upon ultraviolet irradiation of 7-dehydrocholesterol. The cholecalciferol is transported to the liver where it is hydroxylated to produce 25-hydroxycholecalciferol (25-(OH)D₃). The 25-(OH)D₃ has limited biological activity, but in the kidney it undergoes further hydroxylation to form dihydroxy metabolites, the most potent of which in calcium metabolism is 1,25-(OH)₂D₃. An increase in this vitamin D metabolite results in increased intestinal absorption of calcium, mobilization of calcium and phosphorous from the bone, and increased calcium reabsorption in the kidney, all acting to elevate plasma calcium concentrations. The production of this dihydroxy metabolite of vitamin D is regulated by the need for calcium in the circulation. Decreased blood calcium results in a stimulation of the parathyroid glands to secrete PTH that leads to the increased production of 1,25-(OH)₂D₃ in the renal proximal tubules. Thus, PTH is responsible for maintaining the necessary levels of calcium in the body by extracting sufficient calcium from the diet, resorbing it from bone, or preventing its excretion through the renal tubules. Figure 22–8 shows the regulation of PTH secretion. Ingested vitamin D₂ is hydroxylated into 25-(OH)D₂ and follows the same metabolism to 1,25-(OH)₂D₂.

Calcitonin has an opposing action to PTH, but in humans it appears to play a minor role in calcium homeostasis. As a drug, its pharmacologic action is more definitive. It inhibits osteoclastic bone resorption. It also decreases renal tubular reabsorption of calcium, and by these mechanisms opposes the action of PTH. Calcitonin synthesis occurs in the parafollicular C cells of the thyroid gland.

Approximately 98% of calcium is present in the body within the bones in the form of hydroxyapatite, a crystal lattice composed of calcium, phosphorus, and hydroxide. Of the calcium not within the bones, about half is present in extracellular fluid and the remainder is present in a variety of tissues, particularly skeletal muscle. Only about 1% of the calcium in the bones is exchangeable with the extracellular fluid, and it is this pool that is most significantly affected by changes in PTH concentration. Calcium exists in the plasma in 3 distinct forms: free or ionized calcium, protein-bound, and complexed with anions. Ionized calcium is the physiologically active form of calcium and accounts for approximately 45% to 50% of the total calcium in the plasma. Another 40% to 45% of calcium in the plasma is bound to plasma proteins. The protein that binds most of this calcium is albumin, but calcium also binds to some globulins. The remaining 5% to 15% of the total calcium forms a complex with a variety of anions. The most commonly found complexes are calcium phosphate and calcium citrate. The distribution of the 3 forms of calcium changes with alterations in pH in the extracellular fluid and with changes in plasma protein concentration. In general, the serum ionized calcium increases in acidosis and decreases in alkalosis because calcium more easily binds proteins under alkalotic conditions. An increase in the concentration of plasma proteins that bind calcium results in a corresponding increase in total calcium, and a decrease in the plasma proteins may result in a decrease in total calcium.

The metabolism of phosphorus is linked to the metabolism of calcium. About 85% of the phosphorus in an adult is present in the bone as part of hydroxyapatite. Most of the remaining phosphorus in the body is within phospholipids, proteins, carbohydrates, nucleotides, and other important biochemical compounds. Phosphorus is present in virtually all foods, and dietary deficiencies do not occur. The phosphorus in the extracellular fluid exists primarily as HPO₄²⁻ and H₂PO₄⁻, which are collectively known as inorganic phosphorus. The relative amounts of these 2 phosphate anions are pH-dependent. Food ingestion can alter the serum inorganic phosphorus concentration significantly, with an increase in serum phosphorus concentration following the ingestion of phosphate-rich food. Because of the rapidly growing skeletal system, phosphorous demands and serum concentrations are significantly higher in children.

Laboratory Tests

Calcium

Whole blood, serum, or plasma specimens can be used for measurement of total calcium levels. For accurate measurement of the ionized or free form of calcium, the specimen must be transported on ice and must not be exposed to air.

Inorganic Phosphorus

About 15% of the inorganic phosphorus, predominantly HPO₄²⁻ and H₂PO₄⁻, in the plasma is protein-bound, and the remainder is free or complexed to another ion. Organic phosphorus (not measured in the assay for inorganic phosphorus) refers to the phosphorus within phospholipids, proteins, carbohydrates, nucleic acids, and other organic substances.

PTH

The most important test in the differential diagnosis of hypercalcemia is the assay for serum PTH. The biological activity of PTH resides in the first 34-amino terminal amino acids.

The intact hormone (iPTH) with 84 amino acids accounts for much of the circulating PTH, but there are many circulating PTH fragments. The assay for iPTH has largely superseded earlier tests that recognize numerous inactive circulating PTH fragments. One fragment of interest is the fragment of PTH representing amino acids 7-84 that is present in high concentrations during renal disease and capable of antagonizing the PTH receptor. Many newer iPTH assays still recognize the 7-84 fragment, along with the intact molecule, and therefore may not be less clinically informative. An assay for whole PTH is available that only recognizes the 1-84 amino acid PTH.

Intraoperative PTH Assay

Primary hyperparathyroidism requiring parathyroidectomy is a challenge because of variability in the location and number of parathyroid glands. Of parathyroid adenomas, 15% to 20% are ectopic, and not adjacent to the thyroid gland, and approximately 5% of patients have 5 parathyroid glands rather than 4. The success of parathyroid surgery has been improved by intraoperative PTH measurement. The relatively short half-life of PTH has allowed for surgeons to measure plasma PTH concentrations before and after excision of parathyroid tumors in surgery. A decrease in PTH of >50% after resection suggests complete resection of the tumor. Use of this intraoperative test has resulted in a higher incidence of complete removal of hypersecreting parathyroid gland tissue, reduced the need for extensive exploration of the neck, and decreased the need for repeat surgery.

Vitamin D

The quantitation of selected vitamin D metabolites is useful in assessing vitamin D metabolism. Vitamin D metabolites of greatest relevance to calcium metabolism include 25-(OH)D₃ (also known as 25-hydroxyvitamin D) and 1,25-(OH)₂D₃ (also known as 1,25-dihydroxyvitamin D). Currently the most commonly ordered test, as a screening test for vitamin D deficiency, is the total 25-(OH) vitamin D, which is the sum of the 25-(OH) vitamin D₂ and 25-(OH) vitamin D₃ concentrations in serum. Recently, tandem mass spectrometry has increased in use and allowed for measurement of vitamin D₂ and vitamin D₃ in either form (ie, 25-(OH) or 1,25-(OH)) in a single test. 25-(OH) vitamin D is the most abundant metabolite of vitamin D, and it has a long half-life. It is the component measured in most immunoassays for vitamin D. In contrast, 1,25-(OH)₂ vitamin D has a much lower serum concentration, and a shorter half-life (4-6 hours).

PTH-related Protein (PTHrP)

This protein, nearly twice the size of PTH, is equipotent with PTH in inducing hypercalcemia. PTHrP is secreted by numerous tumor tissues. Its shared homology allows PTHrP to bind PTH receptors and stimulate renal proximal tubular reabsorption of calcium. The assay to measure PTHrP shows less than 1% cross-reactivity with PTH.

Bone Markers

Markers for bone turnover can be classified into 2 groups, markers of bone formation and markers for bone resorption. Bone markers should not be used as definitive tests for the diagnosis of osteoporosis. Their primary utility is to monitor response to treatment for bone disease. The markers with the most clinical utility are described below.

Bone Formation Markers

Alkaline Phosphatase.

This enzyme is present in a wide variety of tissues, one of which is the bone. Most laboratory assays for alkaline phosphatase (ALP) measure total ALP. The bone-derived fraction of ALP can be differentiated from its isoenzymes in serum by bone-specific ALP immunoassay or based on its instability. Bone ALP is denatured by heat and urea. Falsely elevated results are commonly seen in liver disease.

Osteocalcin.

Serum osteocalcin is a moderately specific marker for bone formation. Serum concentrations are highest in adolescence and in the newborn, when bone growth is most active, and in renal failure due to clearance impairment. The serum osteocalcin concentration rises in women from the 4th to the 10th decade as the bone turnover increases. Menopause induces a marked increase in bone turnover, often with an increase in serum osteocalcin. Although not as sensitive as collagen markers, measurement of osteocalcin can help predict bone loss in postmenopausal women.

Procollagen Type I Intact N-terminal Propeptide (PINP).

PINP, which is formed during collagen synthesis, is the most sensitive marker of bone formation. Measurement of PINP by radioimmunoassay in serum is recommended for monitoring of therapy to bone disease. It should be measured prior to initiation of therapy and then subsequently 3 to 6 months later. PINP exhibits less intraindividual biovariability than other collagen markers.

Bone Resorption Markers

N- and C-terminal Telopeptide of Type 1 Collagen (NTx and CTx).

NTx and CTx are peptide fragments formed during bone resorption through proteolytic processing of the N- and C-terminal ends of type I collagen, respectively. These can be measured by immunoassay in both serum and urine to assess response to treatment of bone disease. Significant intraindividual variability exists in CTx concentrations because it is affected by diet, exercise, and time of day. NTx should be measured prior to initiation of therapy and then 3 to 6 months later to assess bone disease status.

Pyridinium Cross-links.

Pyridinium cross-links, including deoxypyridinoline (DPD), are a group of products formed during bone resorption as a result of collagen breakdown. These can be measured by immunoassay and are useful in monitoring therapy. Urine pyridinium cross-link concentrations can determine efficacy of bone disease treatment after as little as 2 months of therapy.

Primary Hyperparathyroidism

Description

In primary hyperparathyroidism, there is excess secretion of PTH in the absence of an appropriate stimulus. The disease affects women about twice as frequently as it affects men, and the incidence increases with age. The majority of cases of primary hyperparathyroidism result from a single parathyroid adenoma, with hyperplasia of the parathyroids and parathyroid carcinoma being less common causes. The hypercalcemia in hyperparathyroidism occurs as a result of the direct action of PTH to increase resorption of bone calcium, PTH-induced renal tubular reabsorption of calcium, and synthesis of 1,25-(OH)₂D₃ that promotes the intestinal absorption of calcium. Primary hyperparathyroidism is often identified in asymptomatic individuals who have an unexpected serum hypercalcemia. Symptomatic patients with primary hyperparathyroidism may present with kidney stones, hypertension, polyuria, chronic constipation, depression, neuromuscular dysfunction, recurrent pancreatitis, peptic ulcer, or an unexplained osteopenia.

Diagnosis

Primary hyperparathyroidism may be suspected in patients with an isolated elevated total calcium. In order to rule out effects of binding proteins, ionized calcium should also be determined, especially in patients with abnormal serum concentrations of total protein or albumin. As noted earlier, the total serum calcium is 45% to 50% ionized, 40% to 45% protein-bound (mostly to albumin), and 10% to 15% complexed with small inorganic and organic ions. On demonstration of hypercalcemia, serum PTH and fasting serum phosphorus (because the phosphorus concentration in the serum is altered by diet) should be measured. Assays for PTH can measure the intact molecule, carboxy-terminal, or midregion segments. The use of the intact PTH assay is preferred, especially in patients with renal disease because PTH carboxy-terminal fragments can accumulate with decreased renal function. The diagnosis of hyperparathyroidism is made when both persistent hypercalcemia and elevated serum PTH level are demonstrated. The serum inorganic phosphorus may be low or normal in patients with primary hyperparathyroidism. Patients with severe hyperparathyroidism can have bone pain, skeletal deformities, and even bone fractures (Table 22–7).

Secondary Hyperparathyroidism

Description

Secondary hyperparathyroidism occurs when there is chronic hypocalcemia and an excessive compensatory secretion of PTH. Chronic hypocalcemia is often a result of vitamin D deficiency or renal disease with calcium losses into the urine. Inadequate dietary intake of calcium is a rare cause of hypocalcemia. Secondary hyperparathyroidism is often associated with bone disease due to PTH-mediated bone resorption and calcium release.

Diagnosis

In secondary hyperparathyroidism, there is an elevation in the PTH, but unlike primary hyperparathyroidism, the total and ionized calcium in the serum is low or normal. Tests to identify causes of primary hyperparathyroidism, such as a parathyroid adenoma, should have negative results. Tests should be performed to identify the cause of the chronic hypocalcemia leading to secondary hyperparathyroidism. Vitamin D deficiency and renal disease, as noted previously, are the most common causes of chronic hypocalcemia, and these may be diagnosed with the appropriate laboratory assays (Table 22–7).

Hypoparathyroidism

Description

Hypoparathyroidism occurs most frequently with unintentional removal of the parathyroid glands in the surgical excision of the thyroid gland. Other causes of hypoparathyroidism are much less common. Hypocalcemia resulting from the hypoparathyroidism produces characteristic signs and symptoms, including numbness and tingling, and for patients with very low serum calcium levels, convulsions, and muscle spasms.

Diagnosis

In hypoparathyroidism, serum total and ionized calcium concentrations are low, with a low or undetectable serum PTH concentration. There is an elevation in the serum inorganic phosphorus associated with the decrease in serum calcium (Table 22–7).

Pseudohypoparathyroidism

Description

As the name implies, patients with pseudohypoparathyroidism have signs and symptoms that are characteristic of hypoparathyroidism. This disorder results from a resistance of the tissues to the action of PTH and not a PTH deficiency, hence the use of the term “pseudo.”

Diagnosis

Pseudohypoparathyroidism can be distinguished from true hypoparathyroidism by the high concentration of serum PTH, in the presence of a low serum calcium concentration, in patients with the signs and symptoms of hypoparathyroidism. In addition, patients with pseudohypoparathyroidism demonstrate a lack of metabolic response when infused with PTH.

Vitamin D Deficiency

Description

Vitamin D deficiency, a major cause of secondary hyperparathyroidism and hypocalcemia, is caused by insufficient sun exposure, decreased intestinal absorption, insufficient intake, renal or liver failure, and numerous genetic disorders with defects in vitamin D processing, receptors, or binding proteins. Vitamin D deficiency has recently been defined by the Institute of Medicine as total 25-(OH) vitamin D <20 ng/mL (50 nmol/L), but this cutoff is assay-dependent. Based on this cutoff, it is estimated that the prevalence of deficiency ranges from 20% to 100% of the elderly population, and varies among younger individuals by race, age, and sun exposure. Severe vitamin D deficiency in young children results in characteristic skeletal deformities known as rickets. Consistent with secondary hyperparathyroidism, patients with vitamin D deficiency develop bone disease often manifesting in osteomalacia, osteopenia, or osteoporosis. Vitamin D may also have a role in numerous other tissues. Evidence suggests that 1,25-(OH)₂ vitamin D may play a direct or indirect role in immune modulation, blood pressure regulation, insulin production, and cardiac muscle contractility. Vitamin D deficiency may be associated with colon, prostate, and breast cancer, autoimmune disease, diabetes, and cardiovascular disease.

Diagnosis

High-risk patients (pregnant women, elderly, or patients with darker skin pigmentation) should be screened for vitamin D deficiency by measurement of total 25-(OH) vitamin D. Concentrations less than 20 ng/mL or a different lab-defined cutoff indicate deficiency. It is not useful to measure 1,25-(OH)₂ vitamin D in suspected vitamin D deficiency because of its short half-life and tight regulation by numerous molecules. It is useful in the evaluation of patients with rare forms of inherited rickets. The cause, prognosis, and treatment strategies for vitamin D deficiency can be determined by also measuring PTH, magnesium, and phosphorous in plasma.

Hypercalcemia of Malignancy

Description

The most common cause of severe hypercalcemia in an inpatient hospital population is malignancy. Tumors most often associated with hypercalcemia of malignancy include breast carcinoma, multiple myeloma, and lung carcinoma. The serum calcium level may be elevated as a result of osteolysis in the bone from metastases or humoral-induced hypercalcemia. In humoral hypercalcemia of malignancy, tumor production of PTHrP stimulates the PTH receptors to induce hypercalcemia. The elevated calcium signals downregulation of PTH. The assay for PTHrP is potentially useful when malignancy is suspected as a cause of hypercalcemia.

Diagnosis

Hypercalcemia of malignancy must be differentiated from hyperparathyroidism. Patients with hypercalcemia of malignancy will have elevated total and ionized serum calcium, in the presence of suppressed or low PTH. The low PTH value is the differentiating feature of hypercalcemia of malignancy from primary and secondary hyperparathyroidism, which are associated with high concentrations of serum PTH (Table 22–7). For patients in whom humorally induced hypercalcemia is suspected, the most specific confirmatory test is the assay for PTHrP.

Hypocalciuric Hypercalcemia

Description

Familial hypocalciuric hypercalcemia (FHH) is a rare familial disease most often associated with loss of function mutations in the calcium-sensing receptor gene product expressed in the parathyroid glands and kidneys. Normally, this receptor inhibits release of PTH from the parathyroid glands in the presence of high calcium. In the absence of a functioning receptor PTH release is uncontrolled, leading to elevated PTH and hypercalcemia. In the renal tubules, the calcium-sensing receptors inhibit calcium reabsorption in the presence of high calcium. Without this receptor, calcium is continuously reabsorbed and not excreted, accentuating the high serum concentrations and leading to low urine calcium concentrations (hypocalciuria). Patients who are heterozygous for this mutation typically have asymptomatic hypercalcemia, while those with 2 deficient calcium-sensing receptor genes may require parathyroidectomy in infancy.

Diagnosis

Although rare, FHH is important because there is significant overlap in clinical and biochemical parameters with primary hyperparathyroidism. Thus, it is recommended that the diagnostic evaluation include a combination of clinical, biochemical, and genetic tests. Clinically, patients with FHH are usually asymptomatic, while those with primary hyperparathyroidism have symptoms associated with elevated calcium as well as decreased bone density. FHH patients usually have a personal and family history of hypercalcemia while those with hyperparathyroidism may not. The laboratory workup for FHH shows elevated serum calcium and PTH, and usually a reduced urine calcium. There is variability in urine calcium concentrations. Therefore, the calcium:creatinine clearance ratio (CCCR) is the recommended test for identifying FHH. A CCCR <0.01 suggests FHH, while a ratio >0.02 likely represents primary hyperparathyroidism. For patients with a CCCR between 0.01 and 0.02, genetic identification of mutations in the calcium-sensing receptor gene confirms the diagnosis of FHH.

Osteoporosis

Description

Osteoporosis is the most common metabolic disease of the bone associated with decreased bone mass. The causes of osteoporosis are many and varied. Osteoporosis may be primary or secondary. It can occur in association with hyperparathyroidism as described before, as well as with Cushing syndrome, acromegaly, prolonged use of heparin, excess vitamin D intake, and immobilization, among other conditions and disorders.

Bone mineral density (BMD) studies are preferred for diagnosis of primary osteoporosis. BMD estimates obtained by imaging studies are compared with BMD in normal populations to generate a T-score. The WHO defines osteoporosis as a T-score ≤−2.5. T-scores between −1.0 and −2.4 confirm osteopenia. Laboratory testing is preferred for the evaluation of secondary disease. Bone turnover markers can be used to monitor treatment.

Diagnosis

Osteoporosis as a primary disorder is inferred by the absence of another disease known to induce osteoporosis. Primary osteoporosis is generally idiopathic, postmenopausal, or senile. Secondary osteoporosis is established by demonstrating an underlying process or treatment that leads to osteoporosis.

Osteomalacia

Description

Osteomalacia is deficient mineralization of bone that results from disturbances in calcium and phosphorus metabolism. It can result from a nutritional deficiency of vitamin D, defects in vitamin D metabolism or action, defects in mineral metabolism, or disturbances of the bone cells in the bone matrix. When osteomalacia occurs before the cessation of growth, it is known as rickets. Skeletal deformities appear in rickets because of the compensatory overgrowth of epiphyseal cartilage.

Diagnosis

Radiographic studies can demonstrate the disorder. The specific cause for osteomalacia, if it is identified, is generally established with laboratory testing. There are many disorders associated with the decreased mineralization of the bone.

Osteitis Deformans (Also Known as Paget Disease of Bone)

Description

Osteitis deformans is associated with osteoclastic resorption of bone and extensive production of abnormal, poorly mineralized osteoid. This results in a bone that is structurally weak and prone to deformity and fracture. The disorder may involve 1 bone or may be more generalized.

Diagnosis

In osteitis deformans, ALP is significantly elevated, which reflects osteoblastic proliferation in the deformed bone. The serum calcium and inorganic phosphorus concentrations are usually normal, but may be increased in some patients.

Male Physiology and Biochemistry

The male testes serve 2 important functions (Figure 22–9). One is the production of sperm, and the other is the synthesis and secretion of androgens. Sertoli cells within the testes secrete inhibin, and this glycoprotein inhibits the pituitary secretion of follicle-stimulating hormone (FSH). FSH acts on the Sertoli cells to stimulate sperm production and the synthesis of inhibin. Leydig cells in the testes are responsible for the production of androgens. The Leydig cells in the testes receive stimulation by luteinizing hormone (LH) to promote the conversion of cholesterol, through many intermediates, to testosterone. Testosterone, one of the androgens, is important for maturation of sperm, production of male secondary sex characteristics, and providing negative feedback to the anterior pituitary and hypothalamus to reduce the stimulation of the male testes. The hormone secreted by the hypothalamus in the hypothalamic–pituitary–gonadal axis is gonadotropin-releasing hormone (GnRH). GnRH stimulates the release of both LH and FSH from the pituitary in pulsatile patterns. Higher values for LH and FSH are found in the early morning hours.

The androgens are a collection of 19 carbon steroids that produce masculinization and male secondary sex characteristics. The main androgen secreted by the Leydig cells of the testes is testosterone. Other androgens secreted by the testes include androstenedione and DHEA. These compounds can be metabolized to testosterone and dihydrotestosterone (DHT) in target tissues. Circulating testosterone is a precursor to DHT. As previously noted, a number of androgens are secreted by the adrenal glands, including DHEA, DHEA-sulfate (DHEA-S), androstenedione, and androstenediol. Women also produce testosterone, but only 5% to 10% as much as men. Testosterone, as well as androstenedione, can be converted to estrogens. In men approximately 6% to 8% of the testosterone is converted to DHT, but only about 0.3% to estradiol. Most of the testosterone and DHT in the plasma is bound to plasma proteins. Only approximately 3% is free. The 2 major proteins that bind testosterone and DHT are sex hormone-binding globulin (SHBG) and albumin. In men, approximately 45% to 65% of protein-bound testosterone is associated with SHBG and 35% to 50% is bound to albumin. Protein-bound testosterone in women is distributed approximately two thirds on SHBG and one third on albumin. The bioavailable testosterone includes the small fraction that is free and the portion that is weakly bound to albumin. Testosterone binds less efficiently to albumin, and therefore it is available for tissue uptake when associated with this protein. The main excretory metabolites of testosterone, androstenedione, and DHEA are collectively known as 17-ketosteroids that can be quantitated in the urine.

Laboratory Tests

Total Testosterone

Total testosterone measured by immunoassay is a commonly used first-line test in evaluating a suspected gonadal dysfunction in adult males. Total serum testosterone represents both protein-bound and non-protein-bound testosterone. Because it is subject to diurnal variation, testosterone should be measured in the morning.

Free and Weakly Bound Testosterone

This is the bioavailable pool of circulating testosterone. In cases where total testosterone is abnormal, free testosterone can be assessed as a part of a panel that determines bioavailable testosterone and SHBG. Testosterone and SHBG are measured by immunoassay, and concentrations of free and bioavailable testosterone are derived from a mathematical equation based on the constants for the binding of testosterone to albumin and/or SHBG. This assay is not recommended for women and children because the testosterone concentrations are much lower. Therefore, for women and children, testosterone and SHBG are measured by a more complex method, tandem mass spectrometry, and again free and bioavailable testosterone are determined by the same mathematical equation.

SHBG can be measured by immunoassay separately, but the utility is largely in evaluating bioavailable testosterone in men with suspected hypogonadism and women with hyperandrogenism.

Testosterone Precursors and Metabolites

Immunoassays are available for quantitation of DHT, androstenedione, DHEA, DHEA-S, and other related compounds. Large panel testing for adrenal and gonadal steroids measured by LC/MS/MS are also available, especially for the evaluation of suspected congenital adrenal hyperplasia.

DHEA and DHEA-S

Serum concentrations of DHEA and DHEA-S provide an assessment of adrenal androgen production, which may be altered in patients with various conditions, including adrenal hyperplasia, adrenal tumors, delayed puberty, and hirsutism. DHEA is almost entirely derived from the adrenal glands, and DHEA-S in the circulation originates mostly from the adrenal glands, although in men some of it is derived from the testes. DHEA-S is the preferred test for assessing a suspected adrenal androgen abnormality because addition of the sulfate group stabilizes DHEA, leading to higher concentrations and a longer half-life in circulation.

Urinary 17-Ketosteroids

As noted above, the 17-ketosteroids in the urine are a collection of metabolites of androgenic steroids secreted by the testes, the adrenal glands, and, in women, the ovaries. The urine 17-ketosteroid test detects androsterone, DHEA, and several other steroids. However, it does not detect cortisol, estrogens, pregnanediol, testosterone, and DHT because they do not have a ketone functional group. In men, approximately 33% of the urinary 17-ketosteroids represent metabolites of testosterone secreted by the testes, and most of the remaining 17-ketosteroids are derived from steroids generated in the adrenal glands. In women, the 17-ketosteroids are derived almost exclusively from androgens generated in the adrenal glands. The main purpose of measuring these steroid metabolites is to assess androgen production by the adrenal gland. However, in men, a decrease in 17-ketosteroids in the urine may result also from decreased production of testosterone by the testes. Although the urine 17-ketosteroids are sometimes ordered to evaluate male androgenic status, this test does not detect the major androgens—testosterone and DHT. Therefore, if low androgens are suspected, serum testosterone is the preferred test, rather than 17-ketosteroids. Many clinicians now prefer the assessment of serum DHEA-S over urinary 17-ketosteroids for investigation of adrenal androgen production because a 24-hour urine collection is not required and many drugs interfere with the measurement of 17-ketosteroids.

Disorders Affecting Male Reproduction

Description and Diagnosis

Hypogonadotropic Hypogonadism.

Hypogonadotropic hypogonadism in males is associated with absent or decreased function of the testes. If this impairment is manifested early in life, sexual development is retarded. In hypogonadotropic hypogonadism, there is a defect in the hypothalamus or pituitary that reduces normal gonadal stimulation. There are many causes for this abnormality, including panhypopituitarism and GnRH deficiency. A deficiency of GnRH in the hypothalamus is responsible for the most common form of hypogonadotropic hypogonadism, Kallmann syndrome. Hypogonadism in men is diagnosed by at least 2 measurements showing of decreased total testosterone in serum with symptoms of androgen deficiency. If total testosterone results are borderline, hypogonadism can be confirmed by measuring free or bioavailable testosterone. Patients with hypogonadotropic hypogonadism have low testosterone and below normal or inappropriately normal serum concentrations of LH and FSH. Because there are many causes for the disorder, there is much heterogeneity in the severity of these hormonal deficiencies. A clinical picture of sexual infantilism and low levels of LH, FSH, and testosterone in the serum are characteristic features of hypogonadotropic hypogonadism. In order to differentiate pituitary or hypothalamic sources of this disease, prolactin or other measures of pituitary function or imaging studies may be helpful.

Hypergonadotropic Hypogonadism.

This disorder results from a defect in the testes, which may be a result of injury to the testes. There is active stimulation of the testes, but they are unresponsive in this disorder. Apart from testicular injury, getting older is among the commonly encountered causes of hypergonadotropic hypogonadism. The disorder can also result from testicular damage from radiation or chemotherapy.

Patients with hypergonadotropic hypogonadism have elevated concentrations of LH and FSH in the presence of decreased levels of testosterone. When the source of the gonadal failure is unclear, karyotyping may identify chromosomal anomalies as the cause of the testicular abnormality.

Androgen Insensitivity Syndrome (Testicular Feminization Syndrome).

Patients with androgen insensitivity syndrome (AIS), as it is now called, have a severe defect in androgen action, with resistance to the masculinizing effect of the androgenic hormones. This results in a female habitus, with breast tissue and a vagina that ends in a blind pouch, and undescended male testes.

The circulating concentration of testosterone in patients with AIS (Table 22–8) is normal or elevated for a male. An elevation in testosterone can result in estrogen formation in these individuals because testosterone is a precursor for estrogen. The serum concentration of LH is increased, presumably because of resistance to the negative feedback of testosterone within the pituitary and hypothalamus.

Erectile Dysfunction (Formerly Impotence).

There are many causes for the persistent inability to develop or maintain a penile erection sufficient for intercourse and ejaculation. Although psychogenic impotence is the most common (up to 50%), there are many endocrinologic and nonendocrinologic disorders that are associated with impotence. These include vascular disease, diabetes mellitus, hypertension, neoplasms, and adverse drug effects.

An endocrinologic study of the patient may be pursued by measuring the serum testosterone in the early morning, along with LH and FSH, to assess the hypothalamic–pituitary–male gonadal axis for testosterone production. Chapter 19 has additional discussion of this topic.

Female Physiology and Biochemistry

The ovaries function to produce ova and secrete sex hormones, notably estrogens and progestins. Estrogens maintain the female secondary sex characteristics. They are also essential in the regulation of the menstrual cycle, and breast and uterine growth in the maintenance of pregnancy (see Chapter 20 for a discussion on pregnancy). The estrogens have a major impact on calcium metabolism, and the estrogen depletion associated with menopause results in a loss of bone mineral content. Most of the estrogens in the body are secreted by the ovarian follicles and the corpus luteum. During pregnancy, estrogen is also synthesized in the placenta. Only minute quantities are synthesized by the adrenals. The normal human ovary produces estrogens, progestins, and androgens, but the primary products are estradiol and progesterone. More than 20 different estrogens have been identified. Those with clinical importance are estradiol, also known as E₂; estrone, also denoted as E₁; and estriol, that is E₃. Estradiol is derived almost exclusively from the ovaries, and for that reason the serum estradiol level is considered a reflection of ovarian function. In the nonpregnant state, most of the estrogen (microgram quantities) is derived from the ovaries. In pregnant women, the major source of estrogen is the placenta, which secretes estriol as the major product in milligram amounts. Like most other steroid hormones, the vast majority of the circulating estrogen is bound to plasma proteins. More than 95% of circulating estradiol is bound with high affinity to SHBG and, less avidly, to albumin.

Progesterone is a female sex hormone in the progestin family that plays a central role in female reproductive endocrinology. It is involved in regulation of the menstrual cycle and is produced during pregnancy by the placenta. In the nonpregnant state, progesterone is produced largely by the ovary. The adrenal cortex is only a minor source of progesterone production in both sexes, and progesterone is made in very small quantities in the testes in men. More than 90% of the progesterone is protein-bound in the circulation to corticosteroid-binding globulin. Progesterone can be metabolized to 3 groups of metabolites, one of which is the pregnanediols. Urinary pregnanediol concentration can be used as an index of endogenous production of progesterone because it correlates with alterations in progesterone synthesis and metabolism.

There is a tightly coordinated feedback system among the hypothalamus, anterior pituitary, and ovaries in adolescent and adult women to regulate menstruation. Each menstrual cycle consists of a follicular and a luteal phase. Day 1 is the first day of menstrual bleeding. The follicular phase is associated with follicle growth and is the first part of the cycle. Ovulation occurs around day 14 of the menstrual cycle, and the luteal phase follows in the last half of the cycle.

In general, follicular growth in the ovary is stimulated by FSH, and ovulation and progesterone secretion from the developing corpus luteum are driven by LH. During the menstrual cycle:

• FSH increases during the early part of the follicular phase and then declines until ovulation; there is a gradual decrease in FSH through the luteal phase. FSH guides selection of a dominant follicle.

• LH secretion increases around the middle of the follicular phase and just before ovulation; estrogen secretion in the follicular phase stimulates the pituitary to release LH in a surge, with the peak value for LH appearing 10 to 12 hours before ovulation.

• Estradiol concentrations increase as the selected dominant follicle begins to secrete this hormone during midfollicular phase. Its concentrations rapidly rise as the follicle matures. The estradiol concentration then falls abruptly just before ovulation. At ovulation the ovum is released from the follicle. The leftover tissue, called the corpus luteum, is essential for establishing early pregnancy. The corpus luteum secretes estradiol and progesterone to facilitate implantation. If the ovum is not fertilized, the corpus luteum breaks down and estradiol and progesterone synthesis decline after about 14 days. The decline in both hormones signals the beginning of the menstrual cycle.

• Progesterone is at very low concentrations during the follicular phase; with the midcycle surge of LH and ovulation, the corpus luteum secretes progesterone that increases and reaches its peak concentration approximately 8 days after the midcycle LH surge. As the corpus luteum degrades, progesterone concentrations decline to baseline levels at the end of the luteal phase.

Figure 22–9 illustrates the complex relationships in the hypothalamic–pituitary–female gonadal axis.

Laboratory Tests

Estrogens

Serum estrogen levels are represented by the estradiol (E₂) concentration, because estriol (E₃) in a nonpregnant woman is derived almost exclusively from estradiol. In addition, blood estrone (E₁) levels typically parallel estradiol levels throughout the menstrual cycle, but at a lower concentration. Urinary estrogens can be quantitated as total estrogens or as fractionated estrogens with measurement of estradiol, estrone, and estriol. Since estradiol is derived primarily from the ovaries, the estradiol concentration in the urine can provide a more accurate reflection of ovarian function than total urinary estrogen.

Progesterone

The progesterone concentration in serum is a reflection of progesterone production. Assays for urinary progesterone metabolites are used much less frequently to assess progesterone synthesis than tests for serum progesterone.

Endocrinologic Disorders Affecting Female Reproduction

Description and Diagnosis

Healthy women display considerable variations in the length of the menstrual cycle, but most women have cycles between 25 and 30 days in length (see Figure 22–10). The absence of menstrual bleeding is known as amenorrhea. Primary amenorrhea refers to women who have never menstruated, and secondary amenorrhea refers to women in their reproductive years in whom menstruation was present and then ceased for at least 6 months.

Primary Amenorrhea.

Primary amenorrhea is established if spontaneous regular menstruation has not begun by the age of 16 years, with or without the presence of secondary sex characteristics. The list of causes of primary amenorrhea is lengthy. They include lower genitourinary tract defects such as imperforate hymen; a host of ovarian disorders—approximately 40% of females with primary amenorrhea have Turner syndrome (45 X karyotype) or pure gonadal dysgenesis (either a 46 XX or XY karyotype); adrenal disorders such as CAH; thyroid disorders, notably hypothyroidism; pituitary–hypothalamic disorders such as hypopituitarism and Kallmann syndrome (which also affects men); and pregnancy.

Because of the long list of possible causes, the workup for primary amenorrhea should begin with a careful history and physical examination to look for anatomic defects, development of secondary sexual characteristics, and/or a personal or family history of short stature, infertility, and/or amenorrhea. The laboratory evaluation for amenorrhea begins with measurement of hCG to rule out pregnancy. If not pregnant, patients should undergo imaging studies looking for a uterus and gonads. Patients with a detectable uterus should be evaluated for hypothyroidism and hyperprolactinemia (discussed below). High TSH and low free T₄ results suggest the amenorrhea is due to primary hypothyroidism. An elevated prolactin result should prompt a physician to perform an MRI in search of a pituitary adenoma. Patients with normal TSH and prolactin should be evaluated for gonadotropic function by measuring LH and FSH. Estrogen measurements may be helpful to determine the cause of disease. Because of the day-to-day variability in estrogen concentrations, the progestin challenge test may be helpful to establish estrogen reserves and/or etiology of primary amenorrhea. Theoretically, if progesterone is given to an estrogen-primed uterus, withdrawal bleeding (menstruation) will occur. Progesterone is given orally for up to 1 week. Bleeding should occur within 1 week of progesterone withdrawal if the woman's ovaries have produced enough estrogen (>40 pg/mL serum) to prime her uterus.

If the patient has congenital anomalies, a karyotype evaluation to look for cytogenetic abnormalities may be helpful (Table 22–9).

Secondary Amenorrhea.

Secondary amenorrhea is more common than primary amenorrhea and is the absence of regular menstruation for at least 6 months in a woman who has previously had menses. Oligomenorrhea is present if a woman has less than 9 menstrual cycles per year. The causes of secondary amenorrhea include many of those for primary amenorrhea. However, there are a number of conditions associated with secondary amenorrhea that are independent of primary amenorrhea. Most notably, pregnancy is a common cause of amenorrhea and must be considered first in a patient who has stopped menses. An elevated prolactin concentration, which may be induced by a prolactin-secreting tumor, can produce oligomenorrhea or amenorrhea, presumably by inhibition of the release of LH and FSH by the prolactin. Patients with secondary amenorrhea can be divided into those with and without signs of hirsutism and androgen excess. Hirsutism is the excessive growth of terminal hair in women and in children, in a distribution similar to that which occurs in postpubertal men. Causes of hirsutism may be androgen-dependent, with abnormalities often originating in the ovary or the adrenal gland, or androgen-independent, sometimes from antiepileptic medications. Adult women with hirsutism and androgen excess may carry a diagnosis of adult-onset CAH, ACTH-dependent Cushing syndrome, or polycystic ovary syndrome, among other causes.

Because the list of disorders associated with secondary amenorrhea is even longer than the list associated with primary amenorrhea (Table 22–9), the initial evaluation is very broad until the differential diagnosis is narrowed by the results of physical examination, history, and initial radiographic and laboratory studies. The laboratory workup for secondary amenorrhea begins with measurement of hCG to rule out pregnancy. Hypothyroidism and hyperprolactinemia should next be ruled out as the cause of disease by measuring TSH and prolactin. Patients with normal TSH and prolactin should be evaluated for gonadotropic function by measuring LH and FSH.

Estrogen or a progestin stimulation test may be helpful in cases where the cause of amenorrhea is unclear. In women with hirsutism or other signs of hyperandrogenism, a total or free testosterone and DHEA-S should be measured. An elevation of DHEA-S points to an adrenal origin as a cause for the hirsutism. An elevation in testosterone suggests either an adrenal or an ovarian source, or an androgen-secreting tumor outside the adrenal and ovaries.

Growth Hormone/Anterior Pituitary

Physiology and Biochemistry

Growth hormone (GH) is a major product of the pituitary gland. It is a single-chain polypeptide that has structural similarities to prolactin and placental hormones, known as placental lactogens, with which it has overlapping biological activities. GH has secretory spikes, with a half-life of about 20 minutes, which typically occur several hours after meals and exercise. The secretion of GH also rises after the onset of sleep and reaches a peak in deepest sleep. Two hypothalamic factors control the release of GH from the pituitary. Growth hormone-releasing hormone (GHRH) stimulates GH release from the pituitary, and somatostatin (also known as growth hormone-inhibitory hormone [GHIH]) inhibits GH release. The larger influence on the release of GH by the pituitary is the inhibitory action of somatostatin. To promote growth, GH in the circulation binds to target tissues, mostly cartilage, bone, and other soft tissues. GH predominantly exerts its growth effects by stimulating insulin-like growth factors (IGFs) that are produced in the liver and other tissues. Because of its homology to insulin, GH directly affects lipid, carbohydrate, and protein metabolism. IGFs, previously known as somatomedins, also have multiple effects on growth promotion and metabolism. There are a number of IGFs. Unlike most other peptide hormones, IGFs circulate in the blood in a complex with plasma-binding proteins, known as insulin-like growth factor-binding proteins (IGFBPs). The complex physiology of GH signaling is shown in Figure 22–11.

Laboratory Tests

Growth Hormone

Most of the assays for GH are performed using serum, because the concentration of GH in urine is approximately 0.1% of that in serum. Human GH exists in the pituitary gland and in the circulation as a heterogeneous mixture of isoforms. The presence of GH variants in serum can lead to discrepant results among the different assays for GH, although most commercially available GH assays are now calibrated against WHO-standardized materials. Even if the assay problems did not exist, a single random GH measurement is not usually clinically informative because of the diurnal variability and pulsatile secretion of GH by the pituitary gland. Serum concentrations between pulses in healthy individuals are extremely low and may not even be detectable. Provocative tests aimed at testing its stimulation or suppression are usually required to establish GH abnormalities (Table 22–10). A commonly used stimulation test to assess the adequacy of GH secretion is the insulin tolerance test, which produces a transient hypoglycemia to provoke GH release. One GH suppression test involves the ingestion of an oral glucose load, which in healthy individuals suppresses GH secretion from the pituitary.

Insulin-like Growth Factors

IGF-1 circulates in much higher concentrations in plasma than GH, and its secretion is not episodic or diurnal. Therefore, IGF-1 is a good screening test of suspected GH abnormalities and for monitoring therapy in patients with known abnormalities. A single elevated IGF-1 in patients with signs and symptoms of GH excess should be followed with an oral glucose tolerance test (GH suppression testing). A single decreased IGF-1 should prompt treatment in patients with known growth deficiency or an insulin tolerance (or other GH stimulation) test in patients suspected to have GH deficiency. There are marked differences in IGF-1 concentration between adults and children. Therefore, it is very important to establish age-specific reference intervals.

Growth Hormone Excess—Acromegaly and Gigantism

Description

The most common cause of excess GH production is a chromophobe adenoma of the pituitary gland. A prolonged excess of GH results in an overgrowth of the skeleton with acral enlargements, as well as overgrowth of the soft tissues, and these are found in 100% of the patients with this condition. In adults, this condition is known as acromegaly. Because GH has an action on the cartilaginous portion of the bone, GH excess in children before long bone growth is completed results in gigantism.

Diagnosis

The most important requirement for diagnosis of GH excess is a demonstration of unsuppressible GH secretion. Because of the episodic secretion of GH, as many as 10% of patients with active acromegaly have random serum GH concentrations that are within the normal reference interval. An elevated IGF-1 measurement prompts provocative testing and correlates with disease severity. Serum GH concentrations are typically not suppressed by oral glucose loading in patients with acromegaly. Their serum GH concentrations show either no change from baseline or a slight increase. Healthy individuals show marked suppression after oral glucose load. The serum IGF-1 concentration, even as a random test, correlates with the clinical severity of acromegaly better than the test for glucose-induced GH suppression.

Growth Hormone Deficiency

Description

A deficiency of GH may be congenital or acquired. Children who have inadequate GH production or action will not grow to full height. It should be noted that growth retardation is typically not usually caused by GH deficiency. However, children with growth retardation or reduced growth velocity with no obvious explanation should be evaluated for a GH deficiency. In children, causes of GH deficiency include anatomic damage to the pituitary or hypothalamus, isolated GH deficiency in the pituitary, and a combination of pituitary hormone deficiencies from a variety of causes. In adults, the most common causes of GH deficiency are pituitary irradiation and a pituitary adenoma that impairs GH secretion. Since adults are usually at full height, GH deficiency does not change growth velocity. However, it is associated with cardiac abnormalities, increased cholesterol, reduced muscle mass, energy, and bone density, and lean body mass. GH replacement in deficient adults may increase lean body mass and bone density, and reduce cholesterol.

Diagnosis

After ruling out other causes of short stature, patients with signs and symptoms of GH deficiency can be screened by measuring IGF-1; if low, the diagnosis of GH deficiency requires the demonstration of a persistently low GH concentration in 2 different provocative stimulation tests (Table 22–10). The provocations for GH release that can be used include vigorous exercise, deep sleep, and treatment with glucagon, L-DOPA, insulin, or arginine. Insulin and glucagon are the most commonly utilized agents.

Prolactin/Anterior Pituitary

Physiology and Biochemistry

Prolactin is secreted by the anterior lobe of the pituitary gland. It is a polypeptide with 198 amino acid residues that structurally resembles GH, and is secreted by lactotrophic cells in the anterior pituitary. Prolactin production is regulated by stimuli from the hypothalamus, but unlike most other anterior pituitary hormones, its release is primarily controlled by inhibition rather than by stimulation (see Figure 22–12). The primary negative stimulus to the pituitary that limits prolactin secretion is provided by dopamine. There are various stimulatory factors for prolactin release, along with the inhibitory compounds. One stimulatory factor is TRH. Another mechanism to increase prolactin release from the pituitary is to decrease the inhibitory effect of dopamine. A number of medications inhibit dopamine action. Prolactin secretion, like that of several other anterior pituitary hormones, is episodic. Concentrations of prolactin are at their lowest at midday, with the highest values shortly after the onset of sleep. The major physiologic stimulus for prolactin release is suckling of the breast in lactating women. This results in a rise in maternal plasma prolactin concentrations within minutes to initiate milk production. Prolactin controls the initiation and maintenance of lactation only if the breast tissue is appropriately primed by estrogens and other hormones for ductal growth, development of the breast lobular and alveolar system, and the synthesis of specific milk proteins.

Laboratory Tests

Prolactin

As with GH, there is molecular heterogeneity in prolactin with multiple isoforms, which can lead to discrepant results among the immunoassays. A high-molecular-weight isoform, macroprolactin, is a clinically inactive species that contains large aggregates of prolactin with immunoglobulins. It is recognized by many commercial immunoassays and can be the source of hyperprolactinemia. Physicians should check for macroprolactin by precipitation or size exchange chromatography in any patient with an elevated serum prolactin of unknown origin. The best serum specimen is one that is collected 3 to 4 hours after the subject has awakened. It is important to collect the specimen after an overnight fast, when the patient is still resting, because emotional stress, exercise, ambulation, and protein ingestion all elevate the baseline prolactin level. Multiple sampling may be necessary because of the episodic secretion of prolactin.

Hyperprolactinemia

Description.

There are many causes for an elevated prolactin concentrations, one of which is a pituitary adenoma. Pregnancy, chronic renal failure, chest wall trauma, primary hypothyroidism, and a host of medications can also elevate prolactin levels. Elevation of the serum prolactin concentration is associated with many different signs and symptoms. In women, these include anovulation, with or without menstrual irregularity, and amenorrhea. Men with prolactin-secreting pituitary adenomas often present with oligospermia, impotence, or both.

Diagnosis.

A patient with a prolactin-secreting pituitary adenoma generally has a higher elevation of serum prolactin than someone with hyperprolactinemia from a different cause. Because so many medications can provoke prolactin release, and medications are the most common cause of an elevated serum prolactin level, a medication history is very important. Noteworthy medications that can elevate the serum prolactin level include estrogens, dopamine antagonists such as haloperidol, histamine receptor-blocking agents such as cimetidine, and tricyclic antidepressants.

Hypoprolactinemia

Description and Diagnosis.

This condition is not detected unless a woman fails to lactate postpartum. A low prolactin level in a woman in this setting is consistent with hypoprolactinemia.

Antidiuretic Hormone (ADH)/Posterior Pituitary

Physiology and Biochemistry

The posterior pituitary secretes oxytocin and ADH, also known as arginine vasopressin. ADH is a small peptide with 9 amino acids. Oxytocin has a similar structure. Release of hormones from the posterior pituitary into the circulation occurs with stimulation of selected neurons. In the circulation, ADH and oxytocin are usually not bound to carrier proteins.

ADH secretion is triggered by elevated plasma osmolality as well as decreased blood volume or blood pressure (Figure 22–13). Even a small increase in osmolality causes stimulation of ADH release to increase water retention and decrease the plasma osmolality toward normal. ADH induces increased permeability of water in the renal collecting ducts leading to increased water reabsorption and concentration of urine. ADH is also known as vasopressin because it binds to receptors on smooth muscle cells that induce vasoconstriction, thereby increasing the blood pressure and volume. Other nonosmotic stimuli such as pain, stress, sleep, exercise, and a variety of pharmacologic compounds induce ADH secretion from the posterior pituitary as well. Negative feedback for ADH release is provided by atrial natriuretic peptide (ANP). With an increased circulating blood volume or a decreased osmolality, ANP concentrations increase, inducing decreased ADH release. The osmolality of the plasma also impacts the thirst center to coordinate oral intake of water and conservation of water in the kidney.

Laboratory Tests

Antidiuretic Hormone

ADH can be measured using an immunoassay. It is a temperature-sensitive peptide, and plasma testing should be performed within 24 hours after collection. Freezing the specimen stabilizes it for many months. A single ADH measurement is not diagnostic, and the result should be assessed in the context of the results of serum and urine osmolality testing.

Serum and Urine Osmolality

Serum and urine osmolality are measured using a freezing point depression osmometer. Serum osmolality can be estimated by various mathematical formulas using sodium, glucose, and BUN concentrations.

Water Deprivation Test

Patients with suspected diabetes insipidus (DI) are deprived of all fluids until urine osmolality is constant for 3 hours and/or plasma osmolality is greater than the normal range. Once constant, serum osmolality and ADH are measured. ADH is then administered and urine osmolality and volume are measured at 1 and 2 hours post administration.

Polyuria

Description.

A deficiency of ADH or resistance to the action of ADH results in the failure of the renal tubules to reabsorb water and, as a result, an increased amount of water is lost into the urine. Because urine output is dependent on fluid intake, a normal urine output cannot be defined. However, whenever there is more than 2.5 L of urine generated per day, an investigation for a cause of polyuria is usually indicated.

Polyuria can occur from 3 main causes. The first is deficient production of ADH, as occurs in central DI. In this disorder, the pituitary gland fails to secrete normal amounts of ADH in response to stimuli. When the thirst mechanism is normal, increased fluid intake compensates for water lost into the urine and thereby prevents dehydration. Severe dehydration can occur if the thirst center is abnormal, and there is excess water loss into the urine. Congenital disorders of the pituitary or neoplastic diseases, neurological surgery, head trauma, ischemia, and autoimmune disorders account for most of the cases of central DI.

The second cause for polyuria is insensitivity of renal tubules to ADH. This is known as nephrogenic DI and it can be caused by any damage to the renal tubules that impairs water reabsorption.

The third cause of a polyuric state is excessive water intake. This is known as psychogenic or primary polydipsia. In rare cases, hypothalamic disease can affect the thirst center and induce polydipsia. There are also many medications that can affect the thirst center and cause polydipsia.

Diagnosis.

The differential diagnosis of a polyuric state requires measurements of serum and urine osmolality, serum sodium, urine volume, and plasma ADH concentrations. The first step is to document that polyuria exists by establishing that the urine volume exceeds 2.5 L per day. Glycosuria must be excluded as a cause of the polyuria, as hyperglycemia with diabetes mellitus is a common cause of polyuria. Patients with central DI have hypernatremia, high plasma and low urine osmolality, and low or inappropriately normal plasma ADH level because the pituitary is unable to secrete ADH. Patients with nephrogenic DI also have a high serum sodium and osmolality and a low urine osmolality with polyuria and polydipsia. Because of renal insensitivity, they have a normal to high plasma ADH because the hypothalamus generates excess hormone in an attempt to compensate for high plasma osmolality. Patients with primary polydipsia usually have hyponatremia and normal serum and low urine osmolality, with an appropriately low-normal plasma ADH concentration (Table 22–11). The diagnosis of DI can be confirmed with a provocative test called the water deprivation test. After dehydration, a urine osmolality greater than plasma with a low ADH, followed by an increase in urine osmolality of more than 10% in 1 hour following ADH administration, indicates central DI. The excess ADH promotes reabsorption of water by the kidney, resulting in a decreased urine volume and an increased urine osmolality. A high ADH after dehydration, followed by a failure to increase urine osmolality after ADH administration, suggests nephrogenic DI because the defect in this disorder is a failure of the kidney to respond to ADH. After water deprivation, patients with primary polydipsia will have urine osmolality higher than serum and show no increase in urine osmolality after ADH administration.

Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Description

SIADH is an autonomous, sustained synthesis and release of ADH in the absence of stimuli. Thus, plasma ADH concentrations are inappropriately increased relative to the osmolality. There are a number of known causes of SIADH; it is common among patients with pulmonary or central nervous system disorders. Another cause is production of ADH by a malignant tumor, especially a small cell carcinoma of the lung. In addition, there are a number of medications that stimulate the production of ADH. The patient's blood volume is modestly expanded, and serum sodium concentrations may be decreased along with serum osmolality. SIADH is a commonly encountered cause of hyponatremia in hospitalized patients.

Diagnosis

Patients with SIADH usually have a low serum osmolality, a urine osmolality greater than that of serum, and an elevated urine sodium concentration (Table 22–11). There are many causes for hyponatremia other than SIADH, including congestive heart failure, renal insufficiency, nephrotic syndrome, liver cirrhosis, and treatment with medications that stimulate ADH secretion. SIADH is not often assessed by measurement of plasma ADH because the ADH level is not usually necessary to make the diagnosis.

Multiple Endocrine Neoplasia

Description

MEN is a syndrome most often inherited as an autosomal dominant trait. The MEN syndromes are associated with hyperplasia or tumors in multiple endocrine glands at the same time. There have been many recent advances in the understanding of the genetic basis of the different types of MEN.

• Multiple endocrine neoplasia type-1 (MEN 1; Wermer syndrome)—MEN 1 syndrome involves hyperplasia or neoplasms in 1 or more of the following: the parathyroid gland, the pancreatic islet cells, or the anterior pituitary in patients with a known family history of MEN 1. In the absence of a family history of the syndrome, however, at least 2 or more of the primary MEN 1 tumor types must be involved for a diagnosis of MEN 1. The hormonal presentation of MEN 1 is highly variable because the pituitary and the pancreatic islet cells in neoplastic states can secrete many different hormones. Although the prevalence is reported to be 20 to 200 per million live births, it is likely to be greatly underestimated because the clinical expression of MEN 1 varies and often presents with mild symptoms. Patients with MEN 1 usually present in the fourth decade of life. MEN 1 has been linked to a gene mutation in the MEN 1 (menin) gene on chromosome 11.

• MEN 2 (Sipple syndrome)—The most commonly found abnormality in the MEN 2 syndrome is MTC that occurs in over 95% of patients with MEN 2. Pheochromocytoma develops in over 50% of patients with MEN 2, and parathyroid hyperplasia or adenoma produces hyperparathyroidism in 15% to 30% of patients with MEN 2. MEN 2 includes 3 major phenotypes. Over 90% of the cases of MEN 2 are MEN 2A that includes risk of developing MTC, pheochromocytoma, and hyperparathyroidism. The familial cases of MEN 2A are most often diagnosed in the third or fourth decade of life. In MEN 2B, parathyroid disease is rare and there are separate developmental abnormalities such as ganglioneuromatosis and marfanoid habitus, in addition to pheochromocytoma and MTC. MEN 2B generally presents 10 years earlier than MEN 2A. It accounts for approximately 5% of all MEN 2 cases. MEN 2B is usually recognized early in life. The child with MEN 2B has a characteristic facial appearance with a failure to thrive, mucosal neuromas, and constipation or diarrhea due to the ganglioneuromatoses in the gut. The diagnosis can be made conclusively by demonstrating the presence of a mutation in the RET proto-oncogene. The RET proto-oncogene product is a receptor tyrosine kinase that transmits growth and differentiation signals. The third form of MEN 2 is familial MTC in the absence of pheochromocytoma and hyperparathyroidism. This disorder has a later onset than MEN 2A or MEN 2B and usually has a good prognosis. The most common clinical presentation in the patient with medullary carcinoma is a mass in the neck. The diagnosis is most often made by histopathologic review of a specimen acquired by fine needle biopsy.

Diagnosis

Because of the variety of hormonal abnormalities in MEN 1, many different assays are needed to demonstrate hyperplasia or neoplasms of the parathyroid, pancreatic cells, and/or anterior pituitary, all of which may be involved in MEN 1. Genetic mutations in the coding sequence of the RET proto-oncogene are found in the vast majority of patients with MEN 2 (both MEN 2A and MEN 2B) and those with isolated familial MTC. Any first-degree relative of a patient carrying an MEN-associated mutation should also be evaluated. Onset of C-cell hyperplasia and malignancy of the thyroid in patients with a known RET proto-oncogene mutation should be monitored by measuring calcitonin and routine thyroid ultrasounds (see the section “Thyroid”).

Carcinoid Tumors

Description

Carcinoid tumors are the most common of the endocrine tumors. They are generally found in the wall of the gastrointestinal tract, but also can be found in the pancreas, rectum, ovary, and lung. Tumors originating from the primitive foregut include carcinoid of the bronchus, the stomach, the first portion of the duodenum, and the pancreas. These tumors often secrete 5-hydroxytryptophan, histamine, and other peptides. Carcinoid tumors originating from the primitive midgut are those found in the second portion of the duodenum, the jejunum, the ileum, and the ascending colon. These tumors secrete serotonin, also known as 5-hydroxytryptamine, and other peptides. They are associated with the development of carcinoid syndrome, which is characterized by cutaneous flushing, gastrointestinal hypermotility with diarrhea, heart disease, bronchospasm, myopathy, and increased skin pigmentation. Tumors originating from the primitive hindgut include those of the transverse colon, descending colon, and rectum. These tumors are typically silent because they are usually nonsecretory. Therefore, functioning carcinoid tumors are more likely to be detected if they secrete a compound that has biological activity. The serotonin-secreting carcinoid tumors arising from the primitive midgut or foregut are the ones most often detected. Silent carcinoid tumors are most often discovered incidentally at surgery for other disorders in the gastrointestinal tract. Patients with silent carcinoid tumors may have vague abdominal pain that is either undiagnosed or attributed to irritable bowel syndrome.

Diagnosis

Serotonin (5-hydroxytryptamine) is transported in the blood by platelets. It is metabolized to 5-hydroxyindoleacetic acid (5-HIAA). 5-HIAA is quantitatively the principal metabolite of serotonin, and the majority of it is excreted into the urine and can be used as an indicator of serotonin production. Patients with serotonin-secreting carcinoid tumors of midgut origin usually have markedly elevated concentrations of urinary 5-HIAA. If there is a borderline concentration of 5-HIAA in a random or 24-hour urine specimen, a repeat collection should be made with an avoidance of foods or medications that might elevate the 5-HIAA concentrations. Only when the 5-HIAA is normal or borderline is the measurement of serotonin needed to document the diagnosis. Platelets contain almost all the serotonin found in the blood and for that reason, the serotonin is measured in whole blood (with platelets) or in platelet-rich plasma.

Functioning foregut tumors may also be detected by the urinary 5-HIAA assay, even though they secrete 5-hydroxytryptophan rather than serotonin. Urinary 5-HIAA is elevated because the 5-hydroxytryptophan released from these tumors is converted to serotonin in other tissues and is subsequently metabolized to 5-HIAA. In addition, urine histamine is generally elevated in patients with functioning foregut carcinoid tumors because these tumors (in contrast to midgut carcinoids) usually produce histamine.