Medical Endocrinology 7

medical endocrinology 7



medical endocrinology 7
thyroid-stimulating hormone
thyroid-stimulating hormone (tsh) stimulates the synthesis and secretion of the two thyroid hormones, triiodothyronine (t3) and thyroxine (t4), both produced by the thyroid gland. thyrotropin-releasing hormone (trh) from the hypothalamus controls tsh secretion. release of trh in turn depends on blood levels of t3 and t4 high levels of t3 and t4 inhibit secretion of trh via negative feedback. there is no thyrotropininhibiting hormone (see figure).

figure : regulation of secretion and actions of thyroid hormones. trh _ thyrotropin-releasing hormone, tsh = thyroid-stimulating hormone, t3 = triiodothyronine, and t4 = thyroxine (tetraiodothyronine).
tsh promotes release of thyroid hormones (t3 and t4) by the thyroid gland.

follicle-stimulating hormone
in females, the ovaries are the targets for follicle-stimulating hormone (fsh). each month fsh initiates the development of several ovarian follicles, saclike arrangements of secretory cells that surround a developing oocyte. fsh also stimulates follicular cells to secrete estrogens (female sex hormones). in males, fsh stimulates sperm production in the testes. gonadotropinreleasing hormone (gnrh) from the hypothalamus stimulates fsh release. release of gnrh and fsh is suppressed by estrogens in females and by testosterone (the principal male sex hormone) in males through negative feedback systems. there is no gonadotropin-inhibiting hormone.



luteinizing hormone
in females, luteinizing hormone (lh) triggers ovulation, the release of a secondary oocyte (future ovum) by an ovary. lh stimulates formation of the corpus luteum (structure formed after ovulation) in the ovary and the secretion of progesterone (another female sex hormone) by the corpus luteum. together, fsh and lh also stimulate secretion of estrogens by ovarian cells. estrogens and progesterone prepare the uterus for implantation of a fertilized ovum and help prepare the mammary glands for milk secretion. in males, lh stimulates cells in the testes to secrete testosterone. secretion of lh, like that of fsh, is controlled by gonadotropin-releasing hormone (gnrh).



prolactin
prolactin (prl), together with other hormones, initiates and maintains milk secretion by the mammary glands. by itself, prolactin has only a weak effect. only after the mammary glands have been primed by estrogens, progesterone, glucocorticoids, human growth hormone, thyroxine, and insulin, which exert permissive effects, does prl bring about milk secretion.
ejection of milk from the mammary glands depends on the hormone oxytocin, which is released from the posterior pituitary.
together, milk secretion and ejection constitute lactation.
the hypothalamus secretes both inhibitory and excitatory hormones that regulate prolactin secretion. in females, prolactin-inhibiting hormone (pih), which is dopamine, inhibits the release of prolactin from the anterior pituitary most of the time. each month, just before menstruation begins, the secretion of pih diminishes and the blood level of prolactin rises, but not enough to stimulate milk production. breast tenderness just before menstruation may be caused by elevated prolactin. as the menstrual cycle begins anew, pih is again secreted and the prolactin level dropings.
during pregnancy, the prolactin level rises, stimulated by prolactin- releasing hormone (prh) from the hypothalamus. the sucking action of a nursing infant causes a reduction in hypothalamic secretion of pih.
the function of prolactin is not known in males, but its hypersecretion causes erectile dysfunction (impotence, the inability to have an erection of the penis). in females, hypersecretion of prolactin causes galactorrhea (inappropriate lactation) and amenorrhea (absence of menstrual cycles).



adrenocorticotropic hormone
corticotrophs secrete mainly adrenocorticotropic hormone (acth). acth controls the production and secretion of cortisol and other glucocorticoids by the cortex (outer portion) of the adrenal glands. corticotropin-releasing hormone (crh) from the hypothalamus stimulates secretion of acth by corticotrophs. stress-related stimuli, such as low blood glucose or physical trauma, and interleukin-1, a substance produced by macrophages, also stimulate release of acth.
glucocorticoids inhibit crh and acth release via negative feedback.



melanocyte-stimulating hormone
melanocyte-stimulating hormone (msh) increases skin pigmentation in amphibians by stimulating the dispersion of melanin granules in melanocytes. its exact role in humans is unknown, but the presence of msh receptors in the brain suggests it may influence brain activity. there is little circulating msh in humans. however, continued administration of msh for several days does produce a darkening of the skin. excessive levels of corticotropin-releasing hormone (crh) can stimulate msh release dopamine inhibits msh release.





table :summarizes the principal actions of the anterior pituitary hormones.
table 4 : summary of the principal actions of anterior pituitary hormones
hormone and target tissues principal actions
human growthhormone (hgh) or somatotropin stimulates liver, muscle, cartilage, bone, and other tissues to synthesize and secrete insulinlike growthfactors (igfs) igfs promote growth of body cells, protein synthesis, tissue repair, lipolysis, and elevation ofblood glucose concentration.
thyroid-stimulatinghormone (tsh) orthyrotropin stimulates the synthesis and secretion of thyroid hormones by the thyroid gland.
follicle-stimulatinghormone (fsh) in females, initiates development of oocytes and induces ovarian secretion of estrogens.
in males, stimulatestestes to produce sperm.
luteinizinghormone (lh) in females, stimulates secretion of estrogens and progesterone, ovulation, and formation of corpus luteum.
in males, stimulates testes to produce testosterone.
prolactin (prl) together with other hormones, promotes milk secretion by the mammary glands.
adrenocorticotropichormone (acth) orcorticotropin
stimulates secretion of glucocorticoids (mainly cortisol) by the adrenal cortex.
melanocyte-stimulatinghormone (msh) exact role in humans is unknown but may influence brain activity when present in excess, can causedarkening of skin.




thank you
prof. dr. sa ad merza alaraji