endocrine and metabolic disorders
Overview of the Endocrine System
Anterior Pituitary Function
The cells of the anterior lobe (which constitutes 80% of the pituitary by weight) synthesize and release several hormones necessary for normal growth and development and also stimulate the activity of several target glands. ACTH is also known ascorticotropin.Corticotropin-releasing hormone (CRH) is the primary stimulator of ACTH release, butvasopressinplays a role during stress. ACTH induces the adrenal cortex to releasecortisoland several weak androgens, such as dehydroepiandrosterone (DHEA). Circulatingcortisoland other corticosteroids (including exogenous corticosteroids) inhibit the release of CRH and ACTH. The CRH-ACTH-cortisolaxis is a central component of the response to stress. Without ACTH, the adrenal cortex atrophies andcortisolrelease virtually ceases. TSH regulates the structure and function of the thyroid gland and stimulates synthesis and release of thyroid hormones. TSH synthesis and release are stimulated by the hypothalamic hormone thyrotropin-releasing hormone (TRH) and suppressed (by negative feedback) by circulating thyroid hormones. LH and FSH control the production of the sex hormones. Synthesis and release of LH and FSH are stimulated mainly by gonadotropin-releasing hormone (GnRH) and suppressed byestrogenandtestosterone. One factor controlling GnRH release is kisspeptin, a hypothalamic peptide that is triggered by increased leptin levels at puberty. Two gonadal hormones, activin and inhibin, affect only FSH; activin is stimulative, and inhibin is inhibitory. LH and FSH stimulate ovarian follicular development and Female Reproductive Endocrinology . Physiology . GH stimulates somatic growth and regulates metabolism. Growth hormone–releasing hormone (GHRH) is the major stimulator andsomatostatinis the major inhibitor of the synthesis and release of GH. GH controls synthesis ofinsulin-like growth factor 1 (IGF-1, also called somatomedin-C), which largely controls growth. Although IGF-1 is produced by many tissues, the liver is the major source. A variant of IGF-1 is present in muscle, where it plays a role in enhancing muscle strength. It is less under control of GH than is the liver variant. The metabolic effects of GH are biphasic. GH initially exertsinsulin-like effects, increasing glucose uptake in muscle and fat, stimulating amino acid uptake and protein synthesis in liver and muscle, and inhibiting lipolysis in adipose tissue. Several hours later, more profound anti–insulin-like metabolic effects occur. They include inhibition of glucose uptake and use, causing blood glucose and lipolysis to increase, which increases plasma free fatty acids. GH levels increase during fasting, maintaining blood glucose levels and mobilizing fat as an alternative metabolic fuel. Production of GH decreases with aging. Ghrelin, a hormone produced in the fundus of the stomach, promotes GH release from the pituitary, increases food intake, and in animal models improves memory. Prolactin is produced in cells called lactotrophs that constitute about 30% of the cells of the anterior pituitary. The pituitary doubles in size during pregnancy, largely because of hyperplasia and hypertrophy of lactotrophs. In humans, the major function of prolactin is stimulating milk production. Also, prolactin release occurs during sexual activity and stress. Prolactin may be a sensitive indicator of pituitary dysfunction; prolactin is the hormone most frequently produced in excess by pituitary tumors, and it may be one of the hormones to become deficient from infiltrative disease or tumor compression of the pituitary. -LPH), the enkephalins, and the endorphins. POMC and MSH can cause hyperpigmentation of the skin and are only significant clinically in disorders in which ACTH levels are markedly elevated (eg, Addison Disease , Nelson syndrome). The function of 