The major role of ACTH is to stimulate the formation of steroid hormones in the adrenal gland, such as cortisol. Corticotrophs are cells that produce ACTH and make up 10% to 20% of the anterior lobe. Cortisol secretion is regulated by the hypothalamic-pituitary-adrenal axis. Corticotropin-releasing hormone (CRH), made in the hypothalamus, stimulates the release of ACTH.
ADH, or vasopressin, is synthesized in the hypothalamus. The hormone is transported down axons in vesicles to the posterior pituitary where the hormones are stored. Secretion of ADH is highly sensitive to water balance. Once secreted, ADH causes water retention in the kidneys. The hormone binds to receptors in the kidneys and stimulates free water absorption.
The hypothalamus regulates the release of FSH and LH through gonadotropin-releasing hormone (GnRH). In males, LH stimulates the testosterone production. The role of FSH in males remains uncertain, but may work with testosterone for normal sperm production. In females, LH is a major regulator of ovarian hormone synthesis and oocyte maturation. FSH plays a critical role in follicle growth and in regulating estrogen production in the ovary.
As the name implies, GH's major function is growth promotion. Growth hormone releasing hormone (GHRH), secreted by the hypothalamus, induces production and secretion of GH. Somatostatin, also released by the hypothalamus, inhibits GH secretion, and is primarily responsible for the pulsatile secretion of GH. Most of GH's effect occurs through stimulation of insulin-like growth factor-1 (IGF-1, somatomedin C), primarily by the liver. IGF-1 causes growth of muscle, bone, cartilage, protein synthesis, amino acid transportation, DNA and RNA synthesis, and cell proliferation. IGF-1 also supresses the production of GH as a part of a negative feedback loop.
Oxytocin is the only hormone other than antidiuretic hormone to be stored within the posterior lobe of the pituitary gland, and is the most potent hormone to cause contraction of the uterus. Its effects have been utilized for the induction and augmentation of labor, as well as for prevention and treatment of postpartum hemorrhage. Oxytocin is also involved in lactation, causing milk ejection from the breast.
Prolactin is unique among pituitary hormones in that its secretion is spontaneous in the absence of any stimulation from the hypothalamus. The primary mechanism controlling prolactin secretion is tonic inhibition by the hypothalamus via dopamine secretion. In addition, prolactin can be inhibited by somatostatin. Prolactin releasing factors, including thyrotropin-releasing hormone, estrogen, vasoactive intenstinal peptide, and oxycontin all stimulate the production of prolactin. Serum levels range from 4 to 20 Ál/L and are 20% to 30% lower in men than women. During the third trimester of pregnancy, prolactin levels increase up to 300 Ál/L. Prolactin levels fall rapidly after delivery, and return to resting levels within two to three weeks if breast feeding does not occur. Prolactin causes breast enlargement and breast milk production. It also inhibits gonadal (sex hormone) activity by its influence on the hypothalamus, decreasing the release of growth-hormone releasing hormone (GnRH), and subsequently lutenizing hormone.
The production and secretion of TSH is regulated by thyrotropic releasing hormone (TRH), which is produced in the hypothalamus. The main function of TRH is to stimulate TSH release, although TRH can also cause prolactin secretion. TSH leads to increased formation and secretion of the hormone tetraiodothyronine (T4) and, to a lesser degree, triiodothyronine (T3). T4 results in inhibition of both TRH and TSH release as part of a negative feedback loop. T4 is the major hormone secreted by the thyroid gland and is converted to T3, the metabolically active hormone, by target tissues. Thyroid hormone is critical to the development of the brain in children, and in regulating tissue metabolism in adults.