As we have seen, some of the hormones released by the anterior pituitary gland act on endocrine organs that themselves then release hormones. These tropic hormones are TSH, FSH and LH, and ACTH (see Fig. 38.10; Table 38.2). Here, we look at their target organs in more detail.

TSH acts on the thyroid gland, which is located in the front of the neck (see Fig. 38.10; Table 38.2) and leads to the release of two peptide hormones, thyroxine (T4) and triiodothyronine (T3). These peptide hormones are sufficiently hydrophobic that they enter cells, binding to intracellular receptors to increase the resting metabolic rate of cells throughout the body. As levels of T3 and T4 rise, these hormones act as a brake on the release of TSH by the anterior pituitary gland. This regulation of TSH by negative feedback is critical to maintaining stable TSH levels and a stable metabolic state of the body as a whole. Overproduction of thyroid hormones (hyperthyroidism) creates symptoms that reflect an overly active metabolic state (increased appetite and weight loss), whereas thyroid hormone deficiency (hypothyroidism) creates symptoms that reflect a metabolic state that is too low (fatigue and sluggishness). Both conditions are diagnosed by blood tests that monitor circulating levels of thyroid hormones, and both can be treated by medication.

Because the thyroid hormones T3 and T4 require iodine for their formation, individuals who do not acquire enough iodine from their diets produce too little thyroid hormone. Additionally, in cases of iodine insufficiency, the anterior pituitary gland increases its production of TSH because it is no longer receiving negative feedback. Over time, in response to TSH the thyroid gland enlarges to form a goiter, which is observed as an enlargement of the throat. The introduction of iodized salt has eliminated goiter formation and hypothyroidism in many countries, but in many underdeveloped areas these remain significant public health problems.

The gonadotropic hormones are FSH and LH. They target the female and male gonads—the ovaries and testes (see Fig. 38.10; Table 38.2). In response to FSH and LH, the ovaries and testes each secrete sex hormones that regulate their own development as well as the development of secondary sexual characteristics. Female sex hormones include the steroids estrogen and progesterone. The principal male sex hormone is the androgen testosterone. These sex hormones are common to a vast majority of vertebrates, regulating sexual differentiation, gonadal maturation, and reproductive behavior. The role of these sex hormones in regulating reproductive function is discussed in greater detail in Chapter 42.

Testosterone is a naturally occurring anabolic steroid that stimulates the synthesis in the testes of proteins needed for sperm production and the development of male sexual features and body tissue growth, particularly in muscles. Anabolic steroids promote protein synthesis to build body tissues and anabolic metabolism to store energy within cells. A variety of synthetic anabolic steroids that mimic the action of testosterone have been developed to treat muscle wasting due to loss of appetite and diseases such as cancer and AIDS. Anabolic steroids are used illegally by some male and female athletes to promote muscle strengthening, undermining fair competition. The risks to health of continued use of anabolic steroids are considerable: liver damage, heart disease, and high blood pressure. Tissues become damaged because elevated circulating levels of testosterone produced by anabolic steroids inhibits the normal synthesis of tropic hormones by the anterior pituitary gland.

ACTH released by the anterior pituitary gland is also a tropic hormone. As noted in section 38.1 in explaining how amplification strengthens the action of downstream hormones in a signaling pathway, ACTH acts on the cortex (the outer portion) of the paired adrenal glands (see Fig. 38.10; Table 38.2). The adrenal glands are located adjacent to the kidneys (ad- meaning “near” and “renal” referring to the kidneys). In humans, each small adrenal gland lies just above each kidney. During times of stress, such as starvation, fear, or intense physical exertion, ACTH stimulates adrenal cortex cells to secrete cortisol. The release of cortisol in response to stress affects a broad range of bodily functions, including blood-glucose levels (Chapter 41), immune function (Chapter 43), and blood pressure (Chapter 39). Whereas acute (short-term) responses that lead to the release of ACTH and elevated cortisol by the adrenal cortex are healthy physiological responses that enable animals to respond to stressful situations, chronic (long-term) stress often leads to a broad range of unhealthy conditions that include impaired cognitive and immune function, sleep disruption, and fatigue.

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