Basic review: endocrinology of the normal menstrual cycle.

(1) The normal menstrual cycle depends on hormonal relationships between the hypothalamus, pituitary, and ovary. The mechanisms of hormonal control involve both long-looped feedback control (i.e., E2 feedback to the hypothalamus) and local control (i.e., internal ovary). A hormone may have different effects depending on concentration and timing of appearance during the menstrual cycle. (2) There is a single gonadotropin-releasing hormone that governs both FSH and LH release from the pituitary. FSH and LH release is governed by both the concentration and timing of the hypothalamic-releasing factor, GNRH. (3) The follicular or proliferative phase of the menstrual cycle is characterized by the selective development of a dominant follicle. The follicular phase biochemistry is notable for increasing estradiol production, which inhibits GNRH secretion but increases the gonadotropin pool and prepares the follicle for LH influence by stimulating LH receptors. (4) Ovulation occurs secondary to LH surge triggered by increasing levels of E2 acting in a positive feedback loop on the pituitary. (5) Corpus luteum development is signaled by increasing serum progesterone and is largely an autonomous ovarian phenomenon not subject to a great deal of control by hypothalamic or pituitary hormonal controls, because high levels of progesterone inhibit GNRH and gonadotropins.