Progesterone
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Progesterone is a steroid hormone involved in the female menstrual cycle, pregnancy (supports gestation) and embryogenesis of humans and other species. Progesterone belongs to a class of hormones called progestagens, and is the only naturally occurring human progestagen. Progesterone should not be confused with progestins, which are synthetically produced progestagens.
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Progesterone levels vary during the menstrual cycle and it is an ingredient of most birth control pills where it acts by arresting the cycle. Progesterone receptor antagonists, such as RU-486 (Mifepristone), can also be used to prevent conception or induce medical abortions. It has a number of physiological effects, usually to normalize or restore changes to the body caused by estrogen. These effects include normalizing blood clotting and vascular tone, zinc and copper levels, cell oxygen levels, and use of fat stores for energy. Progesterone also assists in thyroid function and bone building by osteoblasts, and appears to prevent endometrial cancer (involving the uterine lining) and breast cancer.
Progesterone is secreted in the second half of the menstrual cycle (after ovulation) by granulosa cells of the corpus luteum. It serves to thicken the uterine lining, preparing the uterus to support the fertilized egg, or zygote. If pregnancy does not occur, progesterone levels will decrease. If conception has occurred, hCG produced by the embryo causes the corpus luteum to maintain progesterone production until the placenta itself can produce enough progesterone. In addition progesterone inhibits lactation during pregnancy. The fall in progesterone levels following delivery is one of the triggers for milk production.
Chemistry
Like other steroids, progesterone consists of four interconnected cyclic hydrocarbons. Progesterone contains ketone and aldehyde functional groups, as well as two methyl branches. Like all steroid hormones, it is hydrophobic. This is mostly due to its lack of very polar functional groups.
Synthesis
Progesterone, like all other steroid hormones, is synthesized from pregnenolone, a derivative of cholesterol. This conversion takes place in two steps. The 3-hydroxyl group is converted to a keto group and the double bond is moved to C-4, from C-5.
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Progesterone is the precursor of all steroid hormones, including cortisol, aldosterol, testosterone, estrone and estradiol.