You are 43, your periods have been weird for six months, and you want to know if this is perimenopause. Or you are 32, sitting in an IVF clinic on cycle day 3 because they asked you to come in fasting. Either way, FSH is on the order. It is the hormone that tells you whether your brain is shouting at your ovaries, and how loudly.
The mechanic: FSH comes from the pituitary, lands on the ovary (or, in men, the testis), and tells follicles to grow. The ovary talks back through estradiol and inhibin B, telling the pituitary to ease up. As the follicle pool shrinks with age, the ovary's voice gets quieter, and FSH climbs in compensation. That climb is what perimenopause looks like on a lab report.
FSH is most useful in pairs. High FSH plus low estradiol = the ovary is the problem. Low FSH plus low estradiol = the pituitary or hypothalamus is the problem. Day 3 FSH paired with day 3 estradiol (and now usually AMH) is the classic ovarian reserve trio. A single random FSH means very little.
What FSH measures
The hypothalamus releases GnRH in pulses. Pulsatile GnRH stimulates the anterior pituitary to release FSH (and LH). FSH then acts on the gonads.
The system is feedback-regulated:
- In women: estradiol and inhibin B from the ovary suppress FSH. As ovaries age and follicle pool shrinks, inhibin B falls, and FSH rises to compensate. This is why FSH climbs across perimenopause.
- In men: inhibin B from Sertoli cells suppresses FSH. Testicular failure raises FSH; intact spermatogenesis keeps it normal.
FSH is pulsatile and varies more than its long half-life suggests. A single FSH value on a random day in a perimenopausal woman has limited prognostic value, sustained elevation matters more.
FSH reference ranges
| Demographic | Low | High | Unit |
|---|---|---|---|
| Women, follicular phase (day 3) | 3.5 | 12.5 | mIU/mL |
| Women, mid-cycle peak | 4.7 | 21.5 | mIU/mL |
| Women, luteal phase | 1.7 | 7.7 | mIU/mL |
| Postmenopausal women | 25 | 135 | mIU/mL |
| Adult men | 1.5 | 12.4 | mIU/mL |
| Diminished ovarian reserve (day 3) | 10 | 25 | mIU/mL |
| Pre-pubertal children | 0 | 5 | mIU/mL |
Sex- and cycle-dependent:
- Women, follicular phase: 3.5–12.5 mIU/mL.
- Women, mid-cycle peak: 4.7–21.5 mIU/mL.
- Women, luteal phase: 1.7–7.7 mIU/mL.
- Day 3 FSH (women): 4–13 mIU/mL is reassuring; 10–15 borderline; above 15 suggests diminished ovarian reserve.
- Postmenopausal women: 25–135 mIU/mL. Sustained values above 25 in a woman with amenorrhea support menopause.
- Adult men: 1.5–12.4 mIU/mL.
- Pre-pubertal children: below 5 mIU/mL.
The transition through perimenopause is gradual. FSH can swing from 10 to 60 and back to 10 in adjacent cycles for a year or two before settling persistently high.
What high FSH means
High FSH is the pituitary shouting at a gonad that is not responding adequately.
- Menopause: the most common cause in women over 45. Sustained FSH above 25 with amenorrhea over 12 months confirms.
- Premature ovarian insufficiency (POI): same picture before age 40. Affects ~1% of women. Causes include autoimmune oophoritis, prior chemotherapy, fragile X carrier status, Turner syndrome, and idiopathic.
- Diminished ovarian reserve: day 3 FSH 10–25 in a reproductive-age woman.
- Primary testicular failure: Klinefelter syndrome, post-orchitis, prior chemotherapy/radiation, post-mumps. FSH typically 15–60.
- Sertoli-cell-only syndrome: high FSH with normal LH and testosterone, azoospermia.
- Pituitary gonadotropin-secreting adenoma: rare; usually large and presents with mass effects.
- Recent ovulation in a young woman: mid-cycle spike up to 20+; not pathologic.
What low FSH means
Low FSH means the pituitary is not signaling, the problem is upstream of the gonads.
- Hypothalamic amenorrhea: energy deficit, low body weight, overtraining, severe stress. FSH and LH both low. Reversible with restoration of nutrition and rest.
- Hyperprolactinemia: high prolactin suppresses GnRH. Common cause of infertility and amenorrhea.
- Pituitary disease: adenoma, infarction (Sheehan's), surgery, radiation. Often part of broader hypopituitarism.
- Anorexia nervosa: severe form of hypothalamic amenorrhea.
- Hemochromatosis: iron deposition in pituitary.
- Kallmann syndrome: congenital GnRH deficiency, often with anosmia.
- Pregnancy: physiologic suppression.
- Combined oral contraceptives: physiologic suppression. Stop pill at least one cycle before fertility testing.
- Polycystic ovary syndrome: FSH usually normal-low; LH/FSH ratio >2.
Day-3 testing and ovarian reserve
"Day 3 FSH" is drawn on cycle days 2–4 and paired with day 3 estradiol. The logic: in the early follicular phase, FSH should be low because a normally functioning ovary has plenty of follicles producing inhibin B and the early estradiol bump. When ovarian reserve is diminished, inhibin B falls and FSH rises in compensation.
Cutoffs for predicting IVF response:
- FSH <10 mIU/mL with E2 <80 pg/mL: reassuring ovarian reserve.
- FSH 10–15 mIU/mL: diminished reserve; reduced response to stimulation likely.
- FSH >15 mIU/mL: markedly diminished reserve.
- FSH >25 mIU/mL with amenorrhea: premature ovarian insufficiency or menopause.
Caveat: a high day 3 estradiol (>80 pg/mL) can suppress FSH into a normal range. Always interpret the two together. AMH is now the more reliable single ovarian reserve marker, it does not vary across the cycle and correlates better with antral follicle count.
FSH for menopause: in symptomatic women over 45, the diagnosis is clinical (12 months amenorrhea, vasomotor symptoms). Lab confirmation is unnecessary and misleading. Under 45, sustained FSH >25 over months supports POI and warrants workup.
Track this biomarker over time in AskAnything.health — upload your lab results and see trends at a glance.
When to act on FSH
- FSH >25 mIU/mL in woman under 40 with amenorrhea: workup for POI: repeat in 4–6 weeks, autoimmune screen, fragile X, karyotype.
- Day 3 FSH 10–15 with day 3 E2 normal: diminished ovarian reserve; counsel on family-planning timing.
- Low FSH with amenorrhea: central workup: prolactin, TSH, head MRI if appropriate, energy availability assessment.
- High FSH in a man with infertility: primary testicular failure; semen analysis, karyotype, Y-chromosome microdeletion testing.
- Low FSH in a man with low testosterone: secondary hypogonadism; check prolactin, MRI if indicated.
This information is for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider about your lab results.
Tests that complete the picture
- LH: paired with FSH; the LH/FSH ratio matters in PCOS.
- Estradiol: required to interpret day 3 FSH correctly.
- AMH: better single ovarian reserve marker.
- Prolactin: common cause of low FSH/LH.
- TSH: thyroid disease causes menstrual irregularity.
- Testosterone: pair with FSH and LH for male hypogonadism workup.
- Inhibin B: alternative ovarian reserve marker; less commonly available.