Disorders stemming from the endocrine system are among the most frequently encountered patient problems. Currently, diabetes (discussed under Pancreas) and thyroid disorders rank within the top ten most encountered diagnoses. On a broader scale, endocrinopathies cross all ages and genders and often manifest as other clinical entities that make diagnosis challenging. Because multiple organ systems are usually involved, endocrinology can seem daunting in an organ-based approach. However, it can be divided into more manageable sections by approaching the topic first by the gland involved, that is, pituitary, adrenal, or thyroid, and then from the clinical perspective of hyperfunction or hypofunction, based on the concentrations of the respective hormones or peptides secreted into the circulation. This chapter will thus be divided by system or gland and subdivided by functional defect. Diagnosis of endocrine disorders makes heavy use of clinical laboratory testing and that emphasis is reflected in this chapter.
QUICK REVIEW Basic Principles of Clinical Endocrinology Glands, Hormones, and Receptors
The key glands of the endocrine system include the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreas, and gonads. These organs synthesize and secrete specific biochemical messengers, known as hormones, into the blood in a synchronized collaboration with the central nervous system (CNS) and the immune system to regulate metabolism, growth, development, and reproduction (Figure 15-1). Other tissues, such as adipose and gut, are also metabolically active and involved in these activities as well. The term hormone, derived from the Greek hormon meaning to set in motion, was chosen because these compounds act on cells some distance from their site of origin (hence the derivation of the word endocrine, from the Greek krino, to separate). Now we know that many hormones also act locally on neighboring cells at the paracrine level, and even on the very cell from which they originate in an autocrine manner.
Several classification schemes of hormones are recognized, but the simplest consists of three broad groups: peptides, steroids, and amino acid derivatives. The peptide hormones represent the largest and most diverse hormone class and include examples such as growth hormone (GH), adrenocorticotropic hormone (ACTH), and insulin. These are typically synthesized and stored for quick release into the circulation when needed, either as specific gene products or through post-translational modification of precursors and are usually water-soluble. All steroid hormones are derived from cholesterol, and as such, are lipophilic. Compounds in this class are not stored, so the rate of synthesis regulates secretion. Examples of steroid hormones include cortisol, aldosterone, and the androgens. The last group, amines, is derived from the amino acid tyrosine and includes the thyroid hormones (thyroxine (T4) and triiodothyronine (T3)) and the catecholamines (epinephrine, norepinephrine, and dopamine). Each of these compounds is stored as granules in the cytoplasm until needed. Hormone Transport—The Role of Proteins
Upon release by the respective endocrine gland, a hormone is transported through the circulation to various tissues. En route to the target tissue the hormone is exposed ...