The role of the endocrine system is to maintain whole body homeostasis. This is accomplished via the coordination of hormonal signaling pathways that regulate cellular activity in target organs throughout the body. Endocrine mechanisms are also concerned with the ability of humans to reproduce, and the sexual maturation required for this function. Classic endocrine glands are scattered throughout the body and secrete hormones into the circulatory system, usually via ductless secretion into the interstitial fluid. Target organs express receptors that bind the specific hormone to initiate a cellular response. The endocrine system can be contrasted with the neural regulation of physiological function that was the focus of the previous section. Endocrine effectors typically provide “broadcast” regulation of multiple tissues and organs simultaneously, with specificity provided for by the expression of relevant receptors. A change in environmental conditions, for example, often calls for an integrated response across many organ systems. Neural regulation, on the other hand, is often exquisitely spatially delimited, such as the ability to contract just a single muscle. Nevertheless, both systems must work collaboratively to allow for minute-to-minute as well as longer term stability of the body’s interior milieu.
Hormones are the soluble messengers of the endocrine system and are classified into steroids, peptides, and amines (see Chapters 1 and 2). Steroid hormones can cross the lipid-containing plasma membrane of cells and usually bind to intracellular receptors. Peptide and amine hormones bind to cell surface receptors. Steroid hormones are produced by the adrenal cortex (chapter 20), the gonads, testes (chapter 23), and ovaries (chapter 22) in addition to steroid hormones that are made by the placenta during pregnancy (chapter 22). Amine hormones are derivatives of the amino acid tyrosine and are made by the thyroid (chapter 19) and the adrenal medulla (chapter 20). Interestingly, the tyrosine-derived thyroid hormone behaves more like a steroid than a peptide hormone by binding to an intracellular receptor. The majority of hormones, however, are peptides and they are usually synthesized as preprohormones before being cleaved first to prohormones in the endoplasmic reticulum and then to the active hormone in secretory vesicles.
Diseases of the endocrine system are numerous. Indeed, endocrine and metabolic disorders are among the most common afflictions in developed countries, particularly when nutrition and access to health care provisions are generous and high risk individuals are identified by regular screening. At least 11 endocrine and metabolic disorders are present in 5% or more of the adult US population, including diabetes mellitus, osteopenia, dyslipidemia, metabolic syndrome, and thyroiditis. For example, type 2 diabetes mellitus is one of the most prevalent endocrine disorders of the 21st century and involves an inability of the body to respond to insulin. The resulting high blood glucose damages many tissues leading to secondary complications (see Chapter 24). In large part, the high and increasing prevalence of diabetes and other metabolic disorders rests on the substantial prevalence of obesity in developed countries, with as many as a third of the US adult population now considered to be obese, and two thirds overweight. Indeed, based on a 2009 report, obesity also affects 28% of US children aged 12–17, and while the current prevalence of type 2 diabetes in children is quite low, this prevalence is accordingly expected to rise. Further, a number of endocrine disorders are more prevalent in specific ethnic groups, or in a particular gender. Overall, the burden of endocrine and metabolic disorders, with their protean manifestations and complications, is a serious public health crisis and even highlights an apparent national shortage of trained endocrinologists. Many endocrine disorders must be managed by primary care physicians as a result.