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Hormonal regulation of the menstrual cycle
Last reviewed: 05.07.2025
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The menstrual cycle reflects the activity of the hypothalamus-pituitary-ovarian system, which manifests itself in structural and functional changes in the reproductive tract: the uterus, fallopian tubes, endometrium, and vagina. Each cycle ends with menstrual bleeding, the first day of which is considered the beginning of the cycle.
During the first part of the menstrual cycle (follicular phase), FSH secreted by the anterior pituitary gland stimulates estradiol production byovarian granulosa cells. FSH and estradiol cause proliferation of these cells, and estradiol secretion increases. These hormones stimulate LH receptors. Estradiol acts on the uterine endometrium, causing it to thicken and become vascularized, thereby preparing it for implantation of the egg. As the follicles mature, the level of inhibin increases in them and in the blood, which has a selective inhibitory effect on FSH secretion.
The peak of estradiol concentration in the blood, which occurs in the middle of the menstrual cycle (day 14), triggers a surge of LH from the pituitary gland. LH stimulates ovulation (release of a mature egg from the follicle). The remaining cells in the postovulatory follicle form the corpus luteum, which begins to secrete progesterone and estradiol. Progesterone has an inhibitory effect on the secretion of inhibin.
During the second, luteal phase, progesterone together with estradiol cause even greater thickening of the endometrium. Increased vascularization of endometrial cells and their differentiation occur, the cells become secretory.
About 1 week after the formation of the corpus luteum, it begins to reverse development and secretes less estradiol and progesterone. By day 28 of the menstrual cycle, the level of ovarian steroids becomes inadequate to support the life of the thickened endometrium and it is destroyed, which leads to menstruation. Bleeding continues for 3-5 days. Low levels of estradiol and progesterone at the end of the cycle relieve (by the principle of negative feedback) the inhibition of GnRH secretion by the hypothalamus. GnRH levels in the hypothalamus increase, which stimulates the secretion of FSH and LH by the pituitary gland, and the menstrual cycle begins again.
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