An FSH test measures the level of follicle-stimulating hormone in your blood. This hormone, made by a small gland at the base of your brain, plays a central role in reproduction: it triggers egg development in women and sperm production in men. Doctors order the test to investigate infertility, irregular periods, signs of menopause, and abnormal puberty timing in children.
What FSH Does in Your Body
FSH is one of two key reproductive hormones (the other is luteinizing hormone, or LH) released by the pituitary gland. In women, FSH stimulates the ovaries to grow and mature eggs each menstrual cycle. In men, it signals the testes to produce sperm. The amount of FSH in your blood rises and falls based on feedback from the ovaries or testes. When those organs aren’t responding well, FSH levels climb because the pituitary gland keeps trying harder. When the pituitary itself isn’t working properly, FSH drops.
This feedback loop is exactly what makes the test so useful. A single blood draw can reveal whether a reproductive problem originates in the ovaries or testes, or further upstream in the brain.
Why Women Get an FSH Test
The most common reasons a woman is asked to take this test include:
- Infertility. FSH helps assess ovarian reserve, which is a rough measure of how many eggs remain and how well the ovaries are functioning. A high FSH level in a woman of childbearing age suggests the ovaries are struggling to respond, which can make conception harder.
- Irregular or missing periods. If your periods have become unpredictable or stopped entirely, FSH levels help narrow down the cause. High levels point toward an ovarian problem, while low levels suggest the issue may be with the pituitary gland or hypothalamus.
- Menopause and perimenopause. FSH rises significantly as the ovaries wind down hormone production. Perimenopause typically begins around age 45, with full menopause (12 consecutive months without a period) arriving around age 50. An FSH level above 30 IU/L, combined with low estrogen, is one indicator that menopause has begun.
For fertility evaluations, the test is typically drawn on day 2 or 3 of the menstrual cycle, when FSH is at its baseline. Testing at other points in the cycle can give misleading results because FSH naturally fluctuates throughout the month.
Why Men Get an FSH Test
In men, FSH testing is most often ordered when a semen analysis shows a low sperm count or when there are other signs of reproductive difficulty. A high FSH level indicates the testes themselves aren’t producing sperm effectively, a condition called primary testicular failure. A low FSH level suggests the problem is with the pituitary gland not sending the right signals.
Normal FSH for adult men falls between 1.5 and 12.4 mIU/mL. Results outside this range help guide the next step in evaluation, whether that involves imaging of the pituitary gland or a closer look at testicular function.
FSH Testing in Children
Children may need an FSH test if puberty arrives unusually early or unusually late. In precocious puberty, where a child develops sexual characteristics before age 8 (girls) or 9 (boys), elevated FSH can confirm that the pituitary gland has switched on prematurely. In delayed puberty, low FSH may point to an underactive pituitary, while high FSH may indicate that the ovaries or testes aren’t responding to normal signals.
What High FSH Levels Mean
Elevated FSH generally signals that the ovaries or testes are underperforming, forcing the pituitary to produce more hormone in an attempt to compensate. Specific conditions linked to high FSH include:
In women, primary ovarian insufficiency (sometimes called premature ovarian failure) is diagnosed when a woman under 40 has missed periods for 4 to 6 months and has two FSH readings above 40 IU/L taken 30 days apart. Turner syndrome, a chromosomal condition where one X chromosome is missing or partially missing, can cause the ovaries to lose their egg supply very early in life, sometimes before a girl even reaches reproductive age. After menopause, FSH levels naturally rise into the range of 25.8 to 134.8 mIU/mL.
In men, high FSH can result from Klinefelter syndrome (an extra X chromosome), testicular damage from infection or injury, or the effects of chemotherapy or radiation on the testes.
What Low FSH Levels Mean
Low FSH points to a problem with the pituitary gland or the hypothalamus, the brain region that controls it. Possible causes include pituitary tumors, head injuries affecting the pituitary, extreme weight loss, or intense physical stress. Because the ovaries and testes depend on FSH to function, low levels can cause infertility in both sexes, along with low estrogen in women or low testosterone in men.
FSH and Polycystic Ovary Syndrome
In PCOS, FSH itself isn’t necessarily abnormal, but the ratio between LH and FSH shifts in a telling way. In healthy women, LH and FSH circulate in roughly equal amounts, with a ratio between 1 and 2. In women with PCOS, LH rises disproportionately, pushing that ratio to 2 or 3. This imbalance disrupts normal ovulation. The elevated LH drives the ovaries to produce excess androgens (male-type hormones), which is why PCOS often comes with acne, excess hair growth, and irregular cycles. Doctors don’t diagnose PCOS on the LH-to-FSH ratio alone, but the pattern supports the diagnosis alongside other findings.
Normal FSH Reference Ranges
Typical values vary by sex and reproductive stage:
- Adult men: 1.5 to 12.4 mIU/mL
- Premenopausal women: 4.7 to 21.5 mIU/mL
- Postmenopausal women: 25.8 to 134.8 mIU/mL
These ranges can differ slightly between labs, so your results will come with a lab-specific reference range printed alongside them. In premenopausal women, the number also shifts throughout the menstrual cycle, peaking just before ovulation. A single result doesn’t always tell the full story, which is why doctors sometimes repeat the test or pair it with other hormone measurements like estrogen or LH.
What to Expect During the Test
An FSH test is a standard blood draw, usually from a vein in your arm. It takes a few minutes and requires no special preparation in most cases, though your doctor may ask you to test on a specific day of your cycle if you’re still menstruating. Hormonal medications, including birth control pills and testosterone therapy, can alter FSH levels, so let your provider know about any medications you’re taking before the test. Results typically come back within a day or two.