Follicle Stimulating Hormone

FSH, short for follicle-stimulating hormone, is the signal your pituitary sends to the ovaries or testes to mature eggs or sperm. When those glands stop responding, the pituitary pushes harder and FSH climbs, so a high FSH is the body's readout that the gonad is running low: menopause and diminished ovarian reserve in women, testicular failure in men. A low FSH points the other way, back to the pituitary that sends the order.

What is FSH and what does it do?

FSH is one of two gonadotropins (hormones that act on the gonads) made by the front of the pituitary, a pea-sized gland under the brain. The hypothalamus releases GnRH (gonadotropin-releasing hormone) in pulses, the pituitary answers with FSH and LH (luteinizing hormone), and those two travel to the ovaries or testes. In women, FSH recruits and grows the follicles that hold eggs each cycle. In men, FSH supports the Sertoli cells that nurse developing sperm.

The gonad talks back. As follicles grow, they release estradiol (the main estrogen) and a protein called inhibin, and both tell the pituitary to ease off. When the ovary or testis runs low on working tissue, that brake disappears: estradiol and inhibin fall, and FSH climbs to compensate. ¹ That loop is why a high FSH reads as a gland that has stopped answering.

What does a high FSH mean?

A high FSH almost always means the ovary or testis is no longer responding well, so the pituitary works harder to get the same job done. What it points to depends on who you are: in women near midlife, the menopause transition; before age 40, primary ovarian insufficiency; in men, primary testicular failure. The value is read next to the gland's output to confirm the gonad, not the signal, is the source.

Pairing FSH with the sex-hormone output is what separates a gland that has failed from a signal that never arrived. ²⁴

FSH and menopause

In women, a persistently high FSH is the clearest lab sign the ovaries have wound down. As the store of follicles runs out, estradiol and inhibin fall, the brake on the pituitary lets go, and FSH climbs, often past 25 IU/L in the late transition and higher after the final period. ¹

One FSH cannot stage perimenopause. Through the transition the number swings from cycle to cycle and can read menopausal one month and normal the next, so a single value is unreliable. ¹ After age 45, menopause is a clinical diagnosis of 12 months without a period, and guidelines advise against using an FSH test to confirm it. ⁵

NICE advises against using an FSH test to diagnose menopause in women over 45, where 12 months without a period is the clinical diagnosis.Paraphrase of NICE guideline NG23

FSH, ovarian reserve, and fertility

In a fertility workup, FSH is drawn early in the cycle, usually day 3, to gauge ovarian reserve, the pool of eggs still in the ovaries. A low day-3 FSH means the ovary responds to a small nudge; a high day-3 FSH means the pituitary is already pushing hard to recruit one follicle, a sign the reserve has shrunk. FSH rises only late in that decline, which makes it a lagging measure. ³

Because FSH moves late, it is not the first-choice test for egg supply.

FSH in men

In men, FSH supports sperm production, so it earns a place in a fertility or low-testosterone workup. A high FSH alongside a low sperm count or low testosterone points to the testis itself failing, damage to the Sertoli cells or seminiferous tubules that make sperm. Klinefelter syndrome, an extra X chromosome, classically shows high LH and FSH with low testosterone. ² A low FSH and LH with low testosterone points upstream to the pituitary instead.

What is a normal FSH level?

Normal depends on sex and, for women, where you are in the cycle, and the numbers vary by lab and assay. As rough orientation:

Those bands are illustrative; your report's own reference range is the one that counts. ⁴

FSH does not follow a simple age curve so much as a reproductive-stage curve. It stays low and cyclic through the reproductive years, becomes erratic in perimenopause (typically the mid-40s to early 50s), then settles high after menopause. In men it drifts up slowly with age as testicular function declines.

How FSH is tested

• A simple blood draw, no fasting required.
• For ovarian-reserve or menopause questions, women have it drawn on day 3 of the cycle (the first day of full flow is day 1).
• Rarely ordered alone: it goes with LH and estradiol in women, or LH and testosterone in men, so the signal is read against the gland's output.
• Combined hormonal contraception (the pill) and menopausal hormone therapy lower FSH and blur the result; a hormonal IUD may also affect it. ⁴
• Commonly covered when there is a fertility or menopause reason to order it, and inexpensive as labs go.

What does a low FSH mean?

A low FSH is less common and points the workup upstream. If the ovaries or testes are underperforming while FSH and LH stay low or unremarkable, the signal from the pituitary or hypothalamus is the likely problem, not the gonad. Causes include a pituitary tumor, very low body weight or heavy endurance training, high prolactin (a hormone that, when raised, quiets the whole ovary or testis signalling loop), or, in women, hormonal contraception. Doctors call this secondary hypogonadism, where the message never arrives, and the focus shifts from the gonad to the brain. Because a pituitary problem can sit behind it, a low FSH with low estrogen or testosterone warrants a clinician's evaluation, including a prolactin check and, when indicated, pituitary imaging; do not self-interpret it. ²⁴

Read on its own, an FSH number is easy to misjudge. Read next to LH, estradiol or testosterone, and your cycle day or menopausal stage, it shows where a reproductive problem starts, and where it does not.

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