What the FSH test shows
FSH — follicle-stimulating hormone, or follitropin — comes from the anterior pituitary gland, not the ovaries or testes. With LH it is a gonadotropin: the pituitary releases it on cue from GnRH in the hypothalamus to drive the gonads, so it reflects the signal reaching the gonads, not the gland itself, as MedlinePlus explains.
What it does differs by sex. In women FSH grows ovarian follicles in the first half of the cycle and prompts them to make estradiol; in men it drives the testes’ Sertoli cells to make sperm. Its partner LH triggers ovulation and testosterone, so the two are ordered together, and the pattern says more than either alone, per StatPearls.
FSH is governed by feedback: estradiol, testosterone and inhibin from healthy gonads switch it off. So it runs high when the ovaries or testes are failing and low when the pituitary itself is quiet.
FSH normal range
FSH is reported in mIU/mL, numerically identical to the SI unit IU/L (1 mIU/mL = 1 IU/L), so US and international reports use the same numbers. In women it swings with the cycle and rises several-fold after menopause.
| Group | Orientation, mIU/mL (= IU/L) |
|---|---|
| Women — follicular phase (day 2–4) | ~3.5–12.5 |
| Women — mid-cycle (ovulation peak) | ~4.7–21.5 |
| Women — luteal phase | ~1.7–7.7 |
| Women — postmenopausal | ~25.8–134.8 |
| Men (adult) | ~1.5–12.4 |
| Children (before puberty) | low — use the lab’s range |
For fertility and ovarian-reserve questions FSH is drawn early, on day 2–4, with estradiol; a raised day-3 value suggests the ovaries are straining to recruit a follicle. The NICE menopause guideline notes that in women over 45 with typical symptoms FSH is not needed to diagnose menopause, though it is key under 40. Ranges depend on the lab, assay, sex and age — read your result against your own report.
Why FSH is high
A high FSH means the pituitary is calling louder because the gonads are not responding. Roughly by frequency:
- Menopause and perimenopause — by far the commonest reason in women. As the ovaries run low on follicles, estradiol and inhibin fall, feedback lifts, and FSH climbs above 25–40 mIU/mL — the hallmark of the transition, per StatPearls.
- Primary ovarian insufficiency (POI) — the same picture before age 40: a high FSH and low estradiol on two samples a month apart, with irregular or absent periods. Because it carries bone and heart risks, it needs prompt evaluation, per StatPearls.
- Primary testicular failure in men — damage to the sperm-making tissue (mumps orchitis, chemo, radiation, undescended testes, trauma, varicocele) raises FSH, a key clue in male infertility.
- Less common — Turner or Klinefelter syndrome from a young age; rarely, an FSH-secreting pituitary tumor.
When is it urgent? A high FSH is rarely an emergency, but a woman under 40 with one should be assessed without long delay. Menopause after 45 is a normal life stage, not a disease — testing there explains symptoms rather than making the diagnosis.
Why FSH is low
A low FSH points the other way: the pituitary or hypothalamus is not sending the signal — hypogonadotropic hypogonadism (low FSH and LH, with low estradiol or testosterone). Common reasons:
- Functional hypothalamic suppression — very low body weight, an eating disorder, heavy training or stress switch off GnRH — a common cause of missed periods in young women.
- High prolactin — a prolactin-secreting tumor, or certain medicines, suppress GnRH and lower FSH and LH.
- Pituitary disease — a tumor, surgery, radiation, or damage after heavy blood loss in childbirth (Sheehan syndrome).
- Exogenous hormones — combined contraception, testosterone or anabolic steroids, and some fertility drugs suppress FSH by design; opioids too.
- PCOS pattern — FSH is often normal or low while LH is relatively high, so the LH-to-FSH ratio, not FSH alone, is the clue.
A congenital form, Kallmann syndrome, pairs low gonadotropins with a poor sense of smell. Because it shifts the search upstream, a low FSH is read with LH, estradiol or testosterone and often prolactin and TSH.
What to test alongside
FSH is read as part of the reproductive axis. For couples planning a pregnancy or anyone with menstrual or fertility concerns, it is usually paired with:
- LH — its partner gonadotropin; the FSH-to-LH pattern separates gonadal failure from PCOS and pituitary causes.
- Estradiol — the ovary’s main output; separates primary from central problems.
- AMH — a cycle-independent measure of ovarian reserve, often preferred over day-3 FSH.
- Prolactin — high levels suppress FSH and explain many low results.
- Progesterone — confirms ovulation.
- Testosterone and SHBG — the male work-up and androgen excess in women.
- TSH — thyroid disease disturbs cycles and fertility.
- hCG — pregnancy is excluded first when periods stop.
What to do about an abnormal result
- Don’t self-diagnose from one number. FSH swings across the cycle and between samples, so a single out-of-range value is not a diagnosis, and no reason to self-treat with hormones.
- Give the timing. In women the result means little without the cycle day and a same-day estradiol; note any contraception, recent pregnancy or medicines.
- Repeat when needed. POI and many pituitary causes are confirmed on two samples about a month apart, not one.
- High FSH: the doctor pairs it with estradiol (women) or a semen analysis and testosterone (men); persistent elevation under 40, or male infertility, prompts referral.
- Low FSH: the work-up centers on prolactin, other pituitary hormones and sometimes imaging.
- Who to see. Start with your primary-care doctor or GP; they refer to a gynecologist, a fertility specialist or an endocrinologist as the pattern dictates.
Mini-FAQ
Does a high FSH mean I’m in menopause?
In a woman over 45 with typical symptoms and no periods, a high FSH supports menopause but isn’t needed to diagnose it. Before age 40 a persistently high FSH instead suggests primary ovarian insufficiency, which needs prompt evaluation.
What is a normal FSH level?
It depends on sex and, in women, the day of the cycle. Early in the cycle most labs use about 3.5–12.5 mIU/mL (the same as IU/L); after menopause FSH rises above roughly 25–40, and adult men sit around 1.5–12.4. Read your result against your own lab’s range.
What does a low FSH mean?
A low FSH usually means the pituitary or hypothalamus is not sending the signal — from stress, very low body weight, heavy exercise, high prolactin or a pituitary problem. Hormonal contraception and some hormone treatments lower it too.
When in the cycle should FSH be tested?
For fertility and ovarian-reserve questions FSH is usually drawn early, on about day 2–4 of the cycle, together with estradiol. Because FSH changes through the month, the value is read against that day’s expected range.
Is FSH or AMH better for checking ovarian reserve?
AMH is more stable and can be measured on any day, so many clinics now favor it, while day-3 FSH still adds useful information. They are often checked together rather than one replacing the other.
