Follicle-Stimulating Hormone (FSH)

Follicle-stimulating hormone (FSH) is a hormone that signals the ovaries to grow follicles, the sacs that hold eggs. The anterior pituitary, the front part of the pituitary gland, releases it. It responds to pulses of GnRH, a signal from the hypothalamus in the brain.

FSH is a glycoprotein gonadotropin, a hormone that acts on the ovaries or testes. Its main role is to recruit follicles and help them mature in the follicular phase, the first half of the cycle.

Each cycle, FSH recruits a group of follicles. One of them usually becomes dominant and goes on to ovulate. This process depends on an FSH rise that begins late in the luteal phase of the previous cycle, well before the period begins.

Basal FSH is a blood test drawn on cycle day 2, 3, or 4. It is a standard part of an ovarian reserve panel. A high basal FSH means the pituitary is working harder than normal to recruit follicles.

This finding shows a weaker ovarian response. It is often an early sign of declining reserve. The definition of "elevated" varies by laboratory and by age. One high value matters less than a level that continues to rise over several cycles.

FSH must be interpreted together with AMH, AFC, and estradiol drawn on the same day. The panel is meaningful only as a whole.

A high estradiol on day 3 can falsely lower FSH. That effect conceals the true state of the reserve. For this reason, a normal FSH with a high estradiol is not reassuring.

Steiner and colleagues followed a large group of women aged 30 to 44. In this age range, markers of low ovarian reserve, including high FSH, were not associated with infertility or a lower monthly chance of pregnancy.¹ The authors caution against using FSH or AMH to test a woman's current fertility.

A single ovarian reserve value describes follicle recruitment. It does not predict whether a woman can conceive. It should never be used alone to dismiss her fertility or to steer her toward IVF.

In men, FSH stimulates Sertoli cells, which support sperm production (spermatogenesis). A high FSH in a man with low sperm counts often signals primary testicular failure, a condition in which the testes themselves function poorly.

That finding changes the entire clinical approach. It is one reason that evaluation of the male partner belongs at the first visit, not as an afterthought.