FSH (Follicle-Stimulating Hormone) — Definition, Normal Levels, Role in Fertility & IVF

FSH (follicle-stimulating hormone) is a hormone produced by the pituitary gland in the brain. It stimulates the ovaries to develop follicles (the fluid-filled sacs containing eggs) each month. In fertility medicine, FSH is tested on Day 2–4 of the menstrual cycle ("Day 3 FSH") as a marker of ovarian reserve. When ovarian reserve declines with age, the pituitary has to work harder to stimulate the ovaries — it produces more FSH. An elevated Day 3 FSH therefore signals a reduction in ovarian reserve.

Day 3 FSH (measured on Day 2–4 of a menstrual cycle) normal ranges: <10 IU/L = good ovarian reserve; 10–15 IU/L = mildly elevated — reduced reserve, may have lower IVF response; 15–25 IU/L = significantly elevated — poor reserve; >25 IU/L = very poor reserve, approaching menopausal range. Important: always interpret Day 3 FSH alongside same-day oestradiol (E2). If E2 is elevated (>80 pg/mL) on Day 3, it suppresses FSH — giving a falsely "normal" FSH reading. Also: FSH can vary between cycles. If it is ever elevated on any cycle, that is significant even if subsequent tests are normal.

Both measure ovarian reserve but in different ways: AMH: produced by small ovarian follicles; stable throughout cycle (test any day); earlier indicator of decline (falls before FSH rises); most useful for predicting IVF egg yield; affected by OCP use. FSH: produced by the pituitary, rises as ovarian reserve falls; must be tested Day 2–4 of cycle; fluctuates cycle to cycle; a late indicator of poor reserve (rises years after AMH starts falling). Best practice: check both. AMH provides the overall picture of reserve; FSH provides information about the pituitary-ovary axis. They give complementary information. If ever discordant (normal AMH but elevated FSH, or vice versa), manage based on the more abnormal result.

Elevated FSH itself is a marker — not a condition to treat directly. The underlying cause is reduced ovarian reserve, and there is no medication that reliably restores ovarian reserve or permanently lowers FSH. Some things that may transiently lower FSH: DHEA supplementation (weak evidence); CoQ10 (weak evidence); weight management; stopping smoking. In IVF, even with elevated FSH, stimulation is possible — higher gonadotropin doses may be needed. Success rates are lower per cycle, but cumulative success with multiple cycles is achievable, especially in women under 40. Donor eggs are the most reliable option when FSH is persistently very elevated (>25 IU/L) and reserve is severely compromised.

In men, FSH stimulates Sertoli cells in the testes to support sperm production (spermatogenesis). Measuring FSH helps determine the cause of azoospermia (no sperm in semen): Elevated FSH (>10–12 IU/L) + azoospermia = primary testicular failure (non-obstructive azoospermia / NOA) — the testes are not producing sperm despite normal pituitary signalling; chances of finding sperm via surgical extraction (TESE) are 40–60%. Normal FSH + azoospermia = obstructive azoospermia (OA) — sperm are being produced but blocked; surgically correctable or sperm retrieved by PESA/TESE. Very low FSH (<1 IU/L) + azoospermia = secondary hypogonadism (HH) — pituitary not signalling; respond well to FSH + hCG injections.