Driven by various releasing hormones from the hypothalamus, the anterior pituitary gland secretes at least six different tropic hormones (see Figure 1). Probably all these tropic hormones and the glands they control affect behavior in some way. Two of these hormones regulate the function of the adrenal cortex and the thyroid gland:
1. Adrenocorticotropic hormone (ACTH) controls the production and release of hormones of the adrenal cortex. The adrenal cortex, in turn, releases steroid hormones, as illustrated in Figure 1. The levels of ACTH and adrenal steroids show a marked daily rhythm (see Chapter 10), and they play a role in stress response (see Chapter 11).
2. Thyroid-stimulating hormone (TSH) increases the release of thyroid hormones from the thyroid gland and markedly affects thyroid gland size.
Two other tropic hormones of the anterior pituitary influence the gonads, and consequently are termed gonadotropins:
3. Follicle-stimulating hormone (FSH) gets its name from its actions in the ovary, where it stimulates the growth and maturation of egg-containing follicles and the secretion of estrogens from the follicles. In males, FSH governs sperm production.
4. Luteinizing hormone (LH) stimulates the follicles of the ovary to rupture, release their eggs, and form into structures called corpora lutea (singular corpus luteum) that secrete the sex steroid hormone progesterone. In males, LH stimulates the testes to produce testosterone. We discuss the gonadal steroid hormones in more detail in the textbook.
The two remaining tropic hormones control milk production and body growth:
5. Prolactin is so named because it promotes lactation in female mammals. But prolactin has a number of roles in addition to its actions on breast tissue. For example, it is closely involved in the parental behavior of a wide variety of vertebrate species.
6. Growth hormone (GH; also known as somatotropin or somatotropic hormone) acts throughout the body to influence the growth of cells and tissues by affecting protein metabolism. GH is released almost exclusively during sleep. Other factors also affect GH secretion. The stomach secretes a hormone, called ghrelin (from the proto-Indo-European root for “grow”), that stimulates the anterior pituitary to release GH (Kojima et al., 1999). Ghrelin is discussed in more detail in Chapter 9. Starvation, vigorous exercise, and intense stress can all profoundly inhibit GH release (see A Step Further 8.3).
Kojima, M., Hosoda, H., Date, Y., Nakazato, M., et al. (1999). Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature 402: 656–660.