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 regulation of parathyroid hormone secretion. [+] Stimulation; [−] inhibition.
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.
An increase in secretion of PTH produces a rise in serum ionized calcium and a decrease in the serum phosphorus concentration. Calcitonin has an opposing action to PTH, but in humans it appears to play a minor role in calcium homeostasis.
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.
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 the level of PTH.
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.
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.
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.
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.
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.
The success of parathyroid surgery has been improved by intraoperative PTH measurement. The intraoperative PTH assay is used to detect decreases in plasma PTH levels following excision of parathyroid tumors in surgery.
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.
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.
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.
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.
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, 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.
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.
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. In the diagnosis of primary hyperparathyroidism, the total serum calcium is the initial test.
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).
Table 22–7Laboratory Evaluation for Hyperparathyroidism and Hypoparathyroidism ||Download (.pdf) Table 22–7 Laboratory Evaluation for Hyperparathyroidism and Hypoparathyroidism
|Laboratory Test ||Primary Hyperparathyroidism ||Secondary Hyperparathyroidism ||Hypoparathyroidism ||Hypercalcemia of Malignancy |
|Serum total or ionized calcium ||Elevated ||Low or normal ||Low ||Elevated |
|Serum intact PTH ||Elevated ||Elevated ||Low or undetectable ||Low or normal |
|Serum PTH-related protein ||Undetectable ||Undetectable ||Undetectable ||Detectable in some cancers |
|Serum 1,25-dihydroxy vitamin D ||May be elevated but not usually required for diagnosis ||May be elevated, normal, or low depending on the blood concentrations of calcium and phosphorus ||Low ||Low or normal |
|Serum phosphorus (inorganic) ||Low or normal ||Normal ||Elevated ||Low or normal |
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.
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 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.
Hypoparathyroidism occurs most frequently with unintentional removal of the parathyroids in the surgical excision of the thyroid gland. In hypoparathyroidism, the total and ionized calcium levels in the serum are low, with a low or undetectable serum PTH concentration.
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).
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.”
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, 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.
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
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.
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.
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.
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.
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 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.
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 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.
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)
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.
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.