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.
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 ...