FSH

FSH is the pituitary's “grow the gametes” signal. As its name suggests, in women it stimulates ovarian follicles — the little sacs that each hold an egg — to mature, and in men it supports the production of sperm.

FSH is a glycoprotein hormone made by gonadotroph cells of the anterior pituitary in response to GnRH from the hypothalamus. It shares this origin and its partner GnRH driver with LH, but the two have different jobs: FSH leans toward gamete development, while LH leans toward steroid production and triggering.

In the ovary, FSH recruits and grows a cohort of follicles early in the menstrual cycle and stimulates the granulosa cells to make estrogen. In the testis, FSH acts on Sertoli cells to nurture developing sperm. In both sexes, the gonads send back a hormone called inhibin that selectively suppresses FSH, fine-tuning gamete output.

FSH is a useful clinical marker. A persistently high FSH, for instance, is a sign that the ovaries are running low on follicles, as happens approaching menopause, because the failing gonad no longer provides enough feedback to hold FSH down.