In the endocrine system, hormones are factors produced by groups of cells clustered in specific tissues, commonly known as glands, and released into the general circulation to affect the function of distant target cells. Because hormones are responsible for the control of a complex metabolic milieu, they, along with the hormone-producing and target cells, participate in intricate regulatory networks (see Chap. 4 and Chap. 5).

An important feature is positive regulation of hormone synthesis and secretion. For example, gonadotropin-releasing hormone (GnRH)from the hypothalamus stimulates production and release of the pituitary gonadotropins. Another common theme is regulation by negative feedback, by which a trophic hormone stimulates the production and secretion of a second hormone in a target cell that acts on the original gland to decrease secretion of the trophic hormone. For example, thyroid-stimulating hormone (TSH) is produced and secreted from the thyrotrope in the anterior pituitary gland. It stimulates the thyroid gland to synthesize and secrete thyroid hormones, which act on the thyrotrope to decrease further production and release of TSH. Thus, hormones may regulate the biosynthesis and release of other hormones. Moreover, hormones may control other cellular activities by determining the amount and activity of other proteins. This regulation occurs largely at the gene transcriptional level, although some regulation occurs at post-transcriptional, translational, and posttranslational levels.

In addition to operating at the broader level of the endocrine system, factors secreted from a given cell can influence cellular activities in adjacent or neighboring cells (i.e., paracrine effects) or within itself (i.e., autocrine effects; see Chap. 1). Intracellular communication effected by hormonal factors is critical for the integrated function of an organism. The pivotal and ubiquitous nature of hormones in these tasks makes understanding their synthesis and release essential.