Follicle-Stimulating Hormone (FSH)

FSH is a pituitary hormone that drives the gonads' growth-and-maintenance function: in women it recruits and matures ovarian follicles each cycle; in men it supports Sertoli cells and spermatogenesis. As ovarian follicle reserves dwindle approaching menopause, the ovary stops responding to FSH effectively, so the pituitary cranks output up — early-follicular FSH >25 mIU/mL is the lab signature of menopause, and a value rising from year to year in a still-cycling woman is the earliest objective marker of declining ovarian reserve. In men, isolated high FSH with normal LH suggests Sertoli cell / spermatogenesis problem (azoospermia, infertility) rather than testosterone deficiency.

• Menopause — early-follicular FSH ≥25 mIU/mL (often >40); paired with absent periods >12 months.
• Diminished ovarian reserve — FSH rising year-on-year in a still-cycling woman; predicts shorter reproductive window.
• Primary ovarian insufficiency (premature menopause) — high FSH under age 40.
• Primary testicular failure — Klinefelter, post-orchitis, post-chemo; FSH typically higher than LH.
• Turner syndrome, Swyer syndrome — gonadal dysgenesis.

• Hypogonadotropic hypogonadism — pituitary tumour, head trauma, congenital.
• Hyperprolactinaemia — high prolactin suppresses FSH and LH together.
• Eating disorders, athletic amenorrhoea — hypothalamic suppression.
• Oral contraceptives — exogenous feedback suppression.
• Late luteal phase — physiologically low; cycle-day context required.