Follicle-stimulating hormone (FSH) is a reproductive hormone made by the pituitary gland, a pea-sized structure at the base of your brain. It plays a central role in fertility for both men and women, driving the processes that produce eggs and sperm. FSH is one of the most commonly tested hormones in bloodwork related to fertility, menstrual irregularities, and menopause.
How FSH Is Produced
FSH production starts with a chain of signals between two parts of your brain and the rest of your body. The hypothalamus, a small region deep in the brain, releases a signaling molecule called gonadotropin-releasing hormone (GnRH). That signal travels a short distance to the anterior pituitary gland, where specialized cells respond by manufacturing and releasing FSH (along with a partner hormone called luteinizing hormone, or LH) into the bloodstream.
FSH is a protein-based hormone made of two linked chains. One chain is shared with several other hormones, including LH and thyroid-stimulating hormone. The second chain is unique to FSH, which is what allows it to bind only to FSH-specific receptors on its target cells in the ovaries or testes.
What FSH Does in Women
In women, FSH is the primary signal that tells the ovaries to begin maturing eggs each menstrual cycle. During the first half of the cycle (the follicular phase), rising FSH levels stimulate several small fluid-filled sacs called follicles to grow. Each follicle contains an immature egg. As these follicles develop, they produce increasing amounts of estrogen.
That rising estrogen eventually feeds back to the brain, telling the pituitary to dial down FSH production. This feedback loop is how the body typically selects just one dominant follicle to continue developing while the rest stop growing. The dominant follicle goes on to release a mature egg at ovulation, triggered by a surge of LH. FSH levels stay relatively low through the second half of the cycle, then begin climbing again as the next cycle starts.
This is why FSH levels in premenopausal women vary depending on where they are in their cycle. The normal range for menstruating women is roughly 4.7 to 21.5 mIU/mL, with the lowest values typically occurring in the second half of the cycle.
What FSH Does in Men
In men, FSH acts on Sertoli cells, which are specialized support cells inside the testes. FSH receptors exist exclusively on Sertoli cells, and when FSH binds to them, it triggers a cascade of activity that supports sperm production. Sertoli cells essentially nurture developing sperm cells, providing the nutrients, structural support, and chemical signals they need to mature.
FSH’s influence on Sertoli cells changes over the course of male development. In early life, FSH primarily drives Sertoli cell multiplication, which determines how many sperm the testes will eventually be able to produce. During puberty, the signal shifts from promoting cell growth to promoting the maturation processes that actually enable sperm production. The normal FSH range for adult men is 1.5 to 12.4 mIU/mL, and levels remain relatively stable day to day, unlike the cyclical pattern in women.
Normal FSH Levels by Age and Sex
Because FSH levels shift across life stages, “normal” depends on who you are and when the test is taken:
- Adult men: 1.5 to 12.4 mIU/mL
- Premenopausal women: 4.7 to 21.5 mIU/mL (varies by cycle phase)
- Postmenopausal women: 25.8 to 134.8 mIU/mL
Ranges can vary slightly between labs, so your results should always be interpreted alongside the specific reference range printed on your lab report.
What High FSH Levels Mean
Elevated FSH almost always means the ovaries or testes aren’t responding the way the brain expects. When the gonads produce less estrogen or testosterone, the pituitary compensates by pumping out more FSH in an attempt to stimulate them harder. It’s a bit like turning up the volume when no one is answering.
In women, the most common reason for high FSH is menopause or the transition leading up to it (perimenopause). FSH levels above 30 mIU/mL are consistent with perimenopause, and postmenopausal women often have levels between 70 and 90 mIU/mL. That said, the FSH test alone is not considered diagnostic for menopause. Clinicians typically use it alongside symptoms and menstrual history.
In younger women, persistently elevated FSH (above 40 mIU/mL on two tests taken a month apart) can indicate primary ovarian insufficiency, sometimes called premature ovarian failure. This condition affects women under 40 and involves the ovaries losing function earlier than expected. Unlike menopause, ovarian function in primary ovarian insufficiency can still fluctuate, meaning some women may occasionally ovulate even after diagnosis.
In men, high FSH can signal that the testes are not producing sperm effectively. This can result from conditions like Klinefelter syndrome (a chromosomal variation), damage to the testes from injury or infection, or the effects of certain medical treatments.
What Low FSH Levels Mean
Low FSH points to a problem at the source: the pituitary gland or the hypothalamus above it. If either structure is damaged or disrupted, it may not send enough FSH into the bloodstream, and the ovaries or testes never get the signal to do their work. This condition is broadly called hypogonadotropic hypogonadism, meaning low reproductive function caused by low signaling hormones.
Several things can cause this. Pituitary adenomas (benign tumors on the pituitary gland) can compress the surrounding tissue and alter hormone output. Brain tumors near the hypothalamus, inflammatory conditions like sarcoidosis, or complications from brain surgery can also reduce FSH production. In some cases, extreme physical stress, severe calorie restriction, or very intense exercise suppress the hypothalamic signals that trigger FSH release.
FSH and Fertility Testing
If you’re undergoing fertility evaluation, your doctor may order a “Day 3 FSH” test, meaning blood is drawn on the second, third, or fourth day of your menstrual cycle. At this point in the cycle, FSH is at its baseline, and an elevated reading can suggest diminished ovarian reserve, meaning fewer eggs remain available.
The American Society for Reproductive Medicine notes that elevated Day 3 FSH (above 10 mIU/mL) is a specific but not very sensitive marker for diminished ovarian reserve. In practical terms, a high result is meaningful, but a normal result doesn’t guarantee everything is fine. Your doctor may also check estrogen levels on the same day. If estrogen is already elevated early in the cycle (above 60 to 80 pg/mL), it can mask a high FSH reading and still indicate reduced ovarian reserve.
One important nuance: ovarian reserve testing tells you something about how many eggs remain and how the ovaries might respond to fertility medications, but it does not reliably predict whether you can get pregnant on your own. A large prospective study of 750 women aged 30 to 44 with no known fertility issues found that those with high FSH values had similar pregnancy rates after 6 and 12 months of trying compared to women with normal levels. Professional guidelines are clear that a low ovarian reserve result should not be used to discourage someone from attempting pregnancy.
FSH and Polycystic Ovary Syndrome
In women with polycystic ovary syndrome (PCOS), the normal balance between FSH and LH is often disrupted. Typically, LH and FSH exist in a roughly 1:1 or 2:1 ratio. In PCOS, LH levels rise while FSH levels stay the same or drop, pushing that ratio to 2:1 or even 3:1. This imbalance contributes to the hallmark features of PCOS: irregular or absent ovulation, excess androgen production, and the characteristic appearance of multiple small follicles on ultrasound.
The LH-to-FSH ratio is not part of the formal diagnostic criteria for PCOS (which rely on irregular periods, signs of excess androgens, and ultrasound findings), but it provides useful supporting information. Research has also shown that this hormonal imbalance exists regardless of body weight. Women with PCOS who have a normal BMI show essentially the same skewed LH-to-FSH ratio as those with a higher BMI, suggesting the hormonal disruption isn’t simply a consequence of excess weight.