Hypothalamus - Pituitary - Thyroid Endocrine Axis
The hormones that make up the HPT axis control the metabolic processes of all cells in the body and are therefore crucial for the organism to function normally. The secretion of TRH from the hypothalamus activates the HPT axis. After reaching the pituitary, TRH stimulates the release of TSH, which in turn promotes the production and release of T4 and T3 by the thyroid gland. Negative feedback effects of T4 and T3 on both the hypothalamus and the pituitary regulate the HPT system.
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The hypothalamus is a small region located within the brain that controls many bodily functions, including eating and drinking, sexual functions and behaviors, blood pressure and heart rate, body temperature maintenance, the sleep-wake cycle, and emotional states (e.g., fear, pain, anger, and pleasure). Hypothalamic hormones play pivotal roles in the regulation of many of those functions. Because the hypothalamus is part of the central nervous system, the hypothalamic hormones actually are produced by nerve cells (i.e., neurons).
The anterior pituitary produces several important hormones that either stimulate target glands (e.g., the adrenal glands, gonads, or thyroid gland) to produce target gland hormones or directly affect target organs. The pituitary hormones include adreno-corticotropic hormone (ACTH); gonadotropins; thyroid-stimulating hormone (TSH), also called thyrotropin; growth hormone (GH); and prolactin.
The first three of those hormones (ACTH, gonadotropins, and TSH) act on other glands.
- ACTH stimulates the adrenal cortex to produce corticosteroid hormones (primarily cortisol) as well as small amounts of female and male sex hormones.
- Gonadotropins comprise two molecules, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These two hormones regulate the production of female and male sex hormones in the ovaries and testes as well as the production of the germ cells (oocyte and spermatozoa).
- TSH stimulates the thyroid gland to produce and release thyroid hormone.
The remaining two pituitary hormones, GH and prolactin, directly affect their target organs.
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