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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. Classic endocrine glands are scattered throughout the body and secrete hormones into the circulatory system. 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 physiologic 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. Neural regulation, on the other hand, is often exquisitely spatially delimited. 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. The majority of hormones are peptides and usually synthesized as preprohormones before being cleaved first to prohormones in the endoplasmic reticulum and then to the active hormone in secretory vesicles.

Endocrine and metabolic disorders are among the most common afflictions in developed countries. 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. In large part, the high and increasing prevalence of diabetes and other metabolic disorders may be related to the substantial prevalence of obesity in developed countries. Overall, the burden of endocrine and metabolic disorders, with their protean manifestations and complications, is a serious public health crisis.

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