FSH, or follicle-stimulating hormone, is a hormone produced by the pituitary gland at the base of your brain. An FSH blood test measures how much of this hormone is circulating in your bloodstream. It’s one of the most common tests ordered when evaluating fertility, irregular periods, or signs of menopause, and it plays a key role in reproduction for both men and women.
What FSH Does in Your Body
FSH is a chemical messenger that travels from a small gland in your brain to your reproductive organs. In women, it triggers the growth and maturation of eggs inside the ovaries each month. In men, it stimulates the production of sperm. Without adequate FSH, the reproductive system essentially stalls.
FSH works alongside another pituitary hormone called luteinizing hormone (LH). Together, these two hormones coordinate the entire monthly cycle in women and maintain steady sperm production in men. Your body regulates FSH through a feedback loop: when sex hormone levels (estrogen in women, testosterone in men) drop, the pituitary ramps up FSH production to compensate. When those hormones are sufficient, FSH dials back down. This feedback system is central to understanding what your test results actually mean.
Why Your Doctor Ordered This Test
FSH testing is typically ordered for a few specific reasons:
- Fertility evaluation. If you’ve been having difficulty conceiving, FSH levels help assess ovarian reserve in women or sperm production capacity in men.
- Irregular or absent periods. FSH can help distinguish between problems originating in the ovaries versus the brain’s pituitary gland.
- Menopause confirmation. Rising FSH levels are one of the hallmarks of menopause and perimenopause.
- Early or delayed puberty. In children, the test helps evaluate whether puberty is progressing on schedule.
- Low testosterone symptoms in men. FSH helps pinpoint whether the issue is in the testes themselves or in the pituitary gland’s signaling.
FSH is rarely interpreted alone. Your provider will often order LH and estradiol (a form of estrogen) at the same time, since the relationship between these hormones tells a much more complete story than any single number.
Normal FSH Ranges
FSH levels are measured in milli-international units per milliliter (mIU/mL). Normal ranges differ by sex, age, and, in women, the phase of the menstrual cycle.
Women (Premenopausal)
FSH fluctuates throughout the month, which is why the timing of the blood draw matters so much. During the follicular phase (the first half of your cycle, before ovulation), normal levels fall between 1.4 and 9.9 mIU/mL. At the ovulatory peak, they rise to roughly 6.2 to 17.2 mIU/mL. During the luteal phase (after ovulation), they drop back to 1.1 to 9.2 mIU/mL.
After menopause, FSH levels climb significantly, often reaching 25 to 135 mIU/mL. This happens because the ovaries are no longer producing enough estrogen to signal the pituitary to stop releasing FSH. The brain keeps sending the signal, but the ovaries can no longer respond, so FSH stays persistently elevated.
Men
In adult men, the normal range is 1.5 to 12.4 mIU/mL. Because men don’t have a monthly hormonal cycle, their FSH levels remain relatively stable from day to day, making the test simpler to time.
Keep in mind that reference ranges can vary slightly between laboratories. Always compare your results to the specific range printed on your lab report.
What High FSH Levels Mean
Elevated FSH generally signals that the reproductive organs aren’t responding adequately, forcing the pituitary gland to work harder. Think of it like turning up the volume on a speaker that isn’t producing enough sound.
In women, high FSH commonly points to diminished ovarian reserve, meaning fewer eggs are available. This is a normal part of aging, but when it happens before age 40, it may indicate primary ovarian insufficiency (sometimes called premature ovarian failure). High FSH is also one of the clearest biochemical markers of menopause and perimenopause. An FSH level consistently above 25 to 30 mIU/mL, combined with absent periods for 12 months, generally confirms menopause.
In men, elevated FSH can indicate that the testes aren’t producing sperm effectively. Possible causes include damage from infection, radiation, or alcohol use. Genetic conditions like Klinefelter syndrome, where a man carries an extra X chromosome, can also drive FSH levels well above normal.
What Low FSH Levels Mean
Low FSH tells a different story. Instead of the reproductive organs underperforming, the problem is upstream: the pituitary gland or the hypothalamus (the brain region that controls the pituitary) isn’t sending enough signal.
In women, low FSH can mean the ovaries never receive the instruction to mature an egg, leading to absent or irregular periods and difficulty conceiving. In men, it can reduce sperm production. Causes of low FSH include pituitary tumors, significant weight loss or eating disorders, extreme physical stress, and certain medications that suppress the hormonal axis. Hormonal birth control and testosterone supplements, for example, deliberately lower FSH as part of their mechanism. If you’re taking either of these, your FSH result will reflect the medication rather than your natural baseline.
When and How the Test Is Done
The test itself is a simple blood draw, typically from a vein in your arm. No fasting is required. Results usually come back within a day or two.
Timing matters enormously for women who are still menstruating. Because FSH fluctuates throughout the cycle, fertility specialists specifically request the blood draw on day 3 of your menstrual cycle (counting the first day of your period as day 1). This day-3 reading provides the most standardized snapshot of your baseline FSH and is the number used to assess ovarian reserve. If your blood is drawn on a random day, the result is harder to interpret, and your provider may ask you to repeat the test at the correct time.
For men and for postmenopausal women, timing is not a concern since their FSH levels don’t cycle monthly. In some cases, your provider may order two or three draws over several weeks to confirm a pattern, since a single abnormal result can sometimes reflect a temporary fluctuation rather than a true problem.
FSH in Fertility Testing
In fertility workups, a day-3 FSH level above roughly 10 mIU/mL starts to raise questions about ovarian reserve, even though it may still fall within the lab’s printed “normal” range. Higher day-3 values, particularly above 15 mIU/mL, suggest the ovaries may respond less robustly to fertility treatments. This doesn’t mean pregnancy is impossible, but it does influence treatment planning and expectations.
FSH is often paired with an anti-Müllerian hormone (AMH) test and an antral follicle count via ultrasound. Together, these three measurements give a much more reliable picture of ovarian reserve than FSH alone. AMH, in particular, is less affected by cycle timing and has become an increasingly important complement to FSH in fertility clinics.
For men with abnormal semen analyses, an elevated FSH paired with low sperm counts typically points to a production problem within the testes. A low FSH with low sperm counts suggests the issue may be hormonal, originating in the pituitary, and potentially treatable with hormone therapy.