Detailed definition
Follicle-stimulating hormone (FSH) is a heterodimeric glycoprotein secreted by gonadotrope cells of the anterior pituitary in response to hypothalamic gonadotropin-releasing hormone (GnRH). In the female reproductive cycle, FSH stimulates the growth of ovarian follicles and the production of estradiol by granulosa cells. Estradiol normally feeds back negatively on the pituitary, keeping FSH within a narrow range. As ovarian reserve declines through the menopause transition, declining estradiol and falling levels of inhibin B reduce that negative feedback, and FSH rises. Persistent serum FSH above 25–30 mIU/mL in the absence of cycling estradiol is consistent with postmenopause. However, in perimenopause FSH can swing dramatically week to week — sometimes returning to premenopausal levels even within a clearly symptomatic perimenopausal woman — which is why a single FSH measurement is generally unreliable for diagnosis. The STRAW+10 staging system explicitly does not require an FSH cutoff to diagnose perimenopause when symptoms and cycle changes are clear.
Why it matters in menopause
Many women are told to "check their hormones" when they show up with perimenopausal symptoms — and a normal FSH is then used to dismiss them. This is a clinical mistake. NAMS, ACOG, and the STRAW+10 framework all state that symptoms plus cycle changes are sufficient for a perimenopause diagnosis without lab testing in the typical-age woman. FSH is most useful when working up suspected POI in women under 40 or evaluating amenorrhea.