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The endocrine system integrates major organ systems with each other and with the nervous system. The endogenous ligands that the endocrine system uses to perform this integrative task are called hormones. Hormones are released from specialized cells, circulate in the blood, and regulate physiologic processes in one or more target organs. In many endocrine systems, several hormones act in series to regulate organ function. The release of one hormone in the series regulates the release of the next hormone. A series of this type provides multiple levels of regulation and integration and also provides the opportunity for negative feedback, in which the last hormone in the series can reduce the production of earlier hormones in the series and thereby regulate its own production (Figure 22–1). The endocrine system provides many useful therapeutic targets and many drugs either mimic or block the effects of naturally occurring hormones.

Figure 22–1.

Multistep hormonal control of the hypothalamic-pituitary-terminal organ cascade with feedback control. Hormones from the terminal (target) organ regulate release of hormones higher up in the cascade. In most cases, the terminal hormone exerts negative feedback effects, although positive feedback systems also occur. Hormones of the adrenal gland depicted are discussed in Chapter 23. ACTH = adrenocorticotropic hormone; ADH = antidiuretic hormone; CNS = central nervous system; FSH = follicle stimulating hormone; LH = luteinizing hormone; TSH = thyroid-stimulating hormone.

This chapter will focus on drugs that regulate three related endocrine systems. These are (1) the hypothalamic-pituitary endocrine system, which exerts control over many integrative functions and other endocrine tissues and interacts directly with the nervous system; (2) the thyroid gland, an essential regulator of growth, development, and normal function of many organ systems; and (3) the gonadal system, which regulates the development and function of reproductive tissues. Separate chapters will cover the pharmacology of drugs that influence the function of hormones produced by the adrenal gland (Chapter 23), hormones that regulate blood glucose (Chapter 24), and those involved with bone mineralization (Chapter 25).

The overall control of metabolism, growth, and reproduction is mediated by a combination of neural and endocrine systems located in the hypothalamus and pituitary gland. The pituitary consists of an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis). The pituitary is connected to the hypothalamus by a stalk of neurosecretory fibers and blood vessels, including a portal venous system that drains the hypothalamus and perfuses the anterior pituitary. The portal venous system carries small regulatory peptide releasing hormones from the hypothalamus to the anterior pituitary. These releasing hormones regulate release of anterior pituitary hormones, which subsequently regulate target tissues throughout the body (Table 22–1). The hormones released from the posterior lobe of the pituitary (oxytocin and vasopressin) are synthesized in the hypothalamus and transported via neurosecretory fibers in the pituitary ...

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