LH	Ovary, Testicle
FSH	Ovary, Testicle
TSH	Thyroid gland

Pituitary secretion of LH and FSH is episodic with secretory bursts occurring approximately every 60 minutes. The gonadotropins stimulate the production of testosterone in the testis and of estradiol and progesterone in the ovary. Testosterone is the principal feedback inhibitor of release of both FSH and LH in men. In women, estradiol feedback inhibits FSH secretion, and both estradiol and progesterone feedback in-inhibits LH secretion. Inhibin, a regulator of FSH secretion, is a peptide hormone produced by the granulosa cells in women and Sertoli cells in men. The dissociations between LH and FSH secretions have been attributed to differential effects of sex steroids on LH and FSH.

The Pituitary and Sex Organs

Menstruation normally occurs every 28 days, the first day of bleeding being designated day 1. Ovulation occurs on day 14 of the cycle. The first phase of the menstrual cycle (luteal phase) last 12 to 14 days; the second phase (follicular phase) is somewhat more variable in duration. During the first few years after menarche there are fre-quent anovulatory intervals and thus irregular cycles. Later, the cycles become more regular with a typical inter-menstrual interval of 28 days.

The synergistic influences of LH and FSH in-crease estrogen production and endometrial development in the follicular phase. The follicle grows and matures during this phase. A progressive increase in circulating LH levels precedes the cycle surge in LH which precipitates ovulation 10 to 12 hours later, followed by an abrupt decrease in LH and estradiol levels. This LH surge is triggered by a complex neuro-endocrine mechanism due to increasing estradiol and progesterone levels. After ovulation, the remaining Graafian follicle cells are transformed into the corpus luteum which is under LH and chorionic gonadotropin stimulation.

The corpus luteum synthesizes progesterone, estradiol, and 17-hydroxyprogesterone. The increased progesterone levels affect the thermoregulatory site in the hypothalamus and increase the core body temperature. Negative feedback from the increased release of estradiol and progesterone from the corpus luteum results in decreased serum LH and FSH levels in the luteal phase. If conception has not occurred, FSH levels increase again at the end of the luteal phase as the progesterone and estradiol levels decrease. The decrease in progesterone and estradiol levels results in shedding of the endometrium—i.e., menstruation. LH pulse frequency is high (90 minutes) and of low amplitude in the follicular phase. The pulse frequency and amplitude increase immediately before ovulation.

During the perimenopausal stage, the circulating level of estradiol decreases and that of FSH increases, and the number of anovulatory cycles increases. Menopause is defined as the permanent cessation of menses, when no functional ovarian follicles remain. FSH and LH levels are high and estradiol level is low.

There is no cyclic variation of gonadotropin or sex steroid secretion in men. Normal maturation of spermatozoa requires sustained gonadotropin secretion. Unlike the eggs in the ovary, all of which are formed at the time of birth, spermatozoa form from primitive spermatogonia in the testicular tubules, which divide throughout reproductive life. The interstitial cells of the testis (Leydig cells) secrete testosterone under the influence of LH. Both FSH and LH are required for the normal maturation of spermatozoa. Testosterone feedback on the gonadotrophs inhibits LH and FSH secretion. There is no "menopause correlate" in men although there is a small increase in LH and FSH starting in the sixth decade.