Follicle Stimulating Hormone (FSH)

Follicle-stimulating hormone (FSH) is a key reproductive hormone produced by the pituitary gland in the brain. In women, FSH stimulates the ovaries to develop follicles — the fluid-filled sacs containing eggs. In men, FSH stimulates the testes to produce sperm. FSH works together with luteinizing hormone (LH) to regulate the reproductive system, and measuring FSH provides crucial information about fertility and reproductive health.

Why does this matter? FSH levels reveal how well the reproductive system is functioning. In women, FSH helps assess ovarian reserve (how many eggs remain), diagnose causes of irregular periods, confirm menopause, and evaluate infertility. In men, FSH helps evaluate sperm production problems and diagnose causes of infertility or testicular dysfunction. Because FSH responds to feedback from the gonads, its level indicates whether reproductive problems originate in the ovaries/testes or in the brain’s control centers.

Order Your FSH Test

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Key Benefits of Testing

FSH testing is fundamental to reproductive health evaluation. In women, it helps assess ovarian reserve and fertility potential, diagnose premature ovarian insufficiency, confirm menopause, and identify causes of menstrual irregularities. In men, it helps evaluate low sperm counts, diagnose testicular failure, and distinguish between different causes of infertility.

FSH’s value lies in what it tells us about the whole reproductive axis. High FSH signals that the pituitary is working hard to stimulate gonads that aren’t responding well. Low FSH suggests the pituitary itself isn’t sending adequate signals. This distinction guides diagnosis and treatment.

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What Does This Test Measure?

The FSH test measures the concentration of follicle-stimulating hormone in your blood. FSH is a gonadotropin — a hormone that acts on the gonads (ovaries or testes). It’s produced by the anterior pituitary gland under control of gonadotropin-releasing hormone (GnRH) from the hypothalamus.

How FSH Works in Women

FSH stimulates ovarian follicles to grow and mature. At the start of each menstrual cycle, rising FSH recruits a group of follicles to begin developing. As follicles grow, they produce estradiol, which feeds back to the pituitary to reduce FSH secretion. Eventually one dominant follicle emerges while others regress. This follicle produces the egg released at ovulation.

FSH levels fluctuate through the menstrual cycle — highest in the early follicular phase (days 2-4), dropping as estradiol rises, briefly spiking with LH at ovulation, then remaining low in the luteal phase. This pattern means timing matters when testing.

As women age and ovarian reserve declines, the ovaries need more FSH stimulation to produce follicles. Rising baseline FSH is one of the earliest signs of diminishing ovarian reserve and approaching menopause.

How FSH Works in Men

In men, FSH acts on Sertoli cells in the testes, which support and nourish developing sperm. FSH is essential for initiating and maintaining sperm production (spermatogenesis). Unlike in women, FSH levels in men don’t cycle but remain relatively stable.

The testes provide feedback to the pituitary through inhibin B, a hormone produced by Sertoli cells. When sperm production is healthy, inhibin B suppresses FSH. When sperm production fails, inhibin B falls and FSH rises as the pituitary tries to stimulate the testes more strongly.

The Feedback System

Understanding FSH requires understanding feedback. The hypothalamus releases GnRH, which tells the pituitary to release FSH and LH. These hormones stimulate the gonads. The gonads then produce sex hormones (estradiol, testosterone) and other signals (inhibin) that feed back to suppress GnRH, FSH, and LH. When gonads fail, this suppressive feedback is lost, and FSH rises. When the hypothalamus or pituitary fails, FSH is low despite the gonads being understimulated.

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Why This Test Matters

For Women

Assesses ovarian reserve. Elevated early-cycle FSH indicates the ovaries need more stimulation to produce follicles — a sign of diminished ovarian reserve. This helps predict fertility potential and response to fertility treatments. However, FSH is just one measure; AMH and antral follicle count provide additional information.

Diagnoses premature ovarian insufficiency. When periods stop before age 40 with elevated FSH, this confirms premature ovarian insufficiency (POI). The high FSH shows the pituitary is trying to stimulate ovaries that aren’t responding.

Confirms menopause. Persistently elevated FSH with low estradiol confirms menopause. This distinguishes menopause from other causes of missed periods in women around menopausal age.

Evaluates menstrual irregularities. FSH helps determine why periods are irregular or absent. High FSH suggests ovarian problems; low or normal FSH with amenorrhea suggests hypothalamic or pituitary causes.

Guides fertility treatment. FSH levels help predict response to ovarian stimulation medications. Women with elevated FSH may need higher medication doses or may have limited response regardless of dose.

For Men

Evaluates sperm production problems. Elevated FSH in men with low sperm count indicates the testes themselves are failing (primary testicular failure). The pituitary is trying to stimulate sperm production but the testes can’t respond.

Distinguishes causes of infertility. Low or normal FSH with low sperm count suggests the problem is in the pituitary or hypothalamus rather than the testes. This distinction matters because pituitary-level problems may be treatable with hormone therapy.

Diagnoses hypogonadism type. Primary hypogonadism (testicular failure) shows high FSH. Secondary hypogonadism (pituitary/hypothalamic failure) shows low or inappropriately normal FSH. The type determines treatment approach.

Value of Testing

FSH is most informative when interpreted alongside other hormones — LH, estradiol (in women), testosterone (in men), and sometimes prolactin. The pattern of multiple hormone levels tells a more complete story than any single test.

For women considering pregnancy in the future, baseline FSH testing (days 2-4) provides valuable information about ovarian reserve before actively trying to conceive. Elevated FSH in your early 30s might prompt earlier family planning decisions. For men with subtle symptoms of low testosterone, FSH helps classify the problem early, when treatment options may be broader.

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What Can Affect FSH Levels?

Causes of High FSH in Women

• Menopause — ovaries no longer respond to FSH
• Perimenopause — FSH begins rising years before final period
• Premature ovarian insufficiency — ovarian failure before age 40
• Diminished ovarian reserve — fewer eggs remaining
• Turner syndrome and other genetic conditions affecting ovaries
• Ovarian damage from surgery, radiation, or chemotherapy
• Autoimmune oophoritis — immune attack on ovaries

Causes of Low FSH in Women

• Hypothalamic amenorrhea — stress, low weight, or excessive exercise suppresses GnRH
• Pituitary disorders — tumors, surgery, or other conditions affecting FSH production
• Hyperprolactinemia — elevated prolactin suppresses GnRH and FSH
• Polycystic ovary syndrome (some cases)
• Pregnancy — FSH is suppressed
• Hormonal contraceptives — suppress pituitary gonadotropins

Causes of High FSH in Men

• Primary testicular failure — testes can’t produce sperm despite stimulation
• Klinefelter syndrome — XXY chromosomes causing testicular dysfunction
• Testicular damage from infection (mumps orchitis), trauma, radiation, or chemotherapy
• Undescended testes (cryptorchidism)
• Age-related testicular decline
• Certain genetic conditions

Causes of Low FSH in Men

• Pituitary disorders — tumors, surgery, trauma affecting FSH production
• Hypothalamic dysfunction
• Hyperprolactinemia
• Kallmann syndrome — genetic condition with GnRH deficiency
• Anabolic steroid use — suppresses pituitary gonadotropins
• Severe illness or malnutrition

Timing Considerations

In premenopausal women, FSH varies with the menstrual cycle. For assessing ovarian reserve, FSH should be measured on days 2-4 of the cycle (early follicular phase). Random testing produces results that are harder to interpret. In men and postmenopausal women, timing is less critical since FSH doesn’t cycle.

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When Should You Get Tested?

For Women

Irregular or absent periods: FSH helps determine whether the problem originates in the ovaries or the pituitary/hypothalamus.

Fertility evaluation: Baseline FSH (days 2-4) is a standard part of fertility workup, helping assess ovarian reserve.

Suspected premature ovarian insufficiency: Menopausal symptoms or absent periods before age 40 warrant FSH testing.

Confirming menopause: When it’s unclear whether symptoms are due to menopause or other causes, FSH and estradiol help clarify.

Before fertility treatment: FSH helps predict response to ovarian stimulation.

For Men

Abnormal semen analysis: Low sperm count or absent sperm warrants FSH testing to help identify the cause.

Suspected hypogonadism: Signs of low testosterone with possible testicular cause.

Delayed puberty: FSH and LH help determine whether delayed puberty is due to constitutional delay or underlying pathology.

Infertility evaluation: FSH is part of male fertility workup alongside testosterone and semen analysis.

Order Your Test

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Understanding Your Results

FSH interpretation requires context — age, sex, menstrual cycle timing (for premenopausal women), symptoms, and other hormone levels all matter.

For Premenopausal Women

Elevated early-cycle FSH indicates diminished ovarian reserve. The ovaries need more stimulation to produce follicles. This suggests reduced fertility potential and predicts poorer response to fertility medications. Very high levels suggest approaching menopause or premature ovarian insufficiency.

Low FSH with absent periods suggests hypothalamic or pituitary dysfunction rather than ovarian failure. Causes include stress, low body weight, excessive exercise, pituitary tumors, or elevated prolactin.

Normal FSH in the early follicular phase is reassuring for ovarian reserve, though it doesn’t guarantee fertility.

For Postmenopausal Women

FSH is consistently elevated after menopause because the ovaries no longer produce hormones that suppress it. Elevated FSH with low estradiol confirms menopausal status.

For Men

Elevated FSH indicates the testes aren’t producing sperm adequately despite pituitary stimulation (primary testicular failure). The higher the FSH, generally the more severe the testicular dysfunction. Very high FSH with absent sperm (azoospermia) suggests severe or irreversible damage.

Low or normal FSH with low sperm count suggests the problem may be at the pituitary or hypothalamus level (secondary hypogonadism). This is potentially treatable with hormonal therapy.

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What to Do About Abnormal Results

For High FSH in Women

Confirm with repeat testing. A single elevated FSH should be confirmed, as levels can fluctuate.

Complete the evaluation with estradiol, LH, and AMH. AMH provides additional information about ovarian reserve that doesn’t fluctuate with the cycle.

If confirmed diminished ovarian reserve, discuss fertility implications. Options may include expedited family planning, fertility treatment, or egg freezing depending on circumstances and goals.

For premature ovarian insufficiency, hormone replacement therapy is typically recommended to protect bone and cardiovascular health until typical menopause age.

For Low FSH in Women

Check prolactin — elevated prolactin suppresses FSH and is treatable.

Evaluate for hypothalamic amenorrhea — assess weight, stress, and exercise patterns. Often reversible with lifestyle changes.

Consider pituitary imaging if other pituitary hormones are also abnormal or if there’s no obvious hypothalamic cause.

For High FSH in Men

Complete evaluation with testosterone, LH, and semen analysis.

Testicular ultrasound may be indicated to evaluate testicular structure.

Genetic testing (karyotype for Klinefelter syndrome) may be appropriate, especially with very high FSH and small testes.

Fertility counseling — high FSH with severe sperm problems may indicate limited fertility options. Donor sperm or advanced reproductive techniques may be discussed.

For Low FSH in Men

Check other pituitary hormones — low FSH may be part of broader pituitary dysfunction.

Pituitary MRI may be indicated to evaluate for tumors or other structural problems.

Treatment options — if pituitary is the problem, gonadotropin therapy (FSH/LH injections) may restore sperm production, unlike primary testicular failure.

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Related Health Conditions

Premature Ovarian Insufficiency

When ovaries stop functioning before age 40, FSH rises as the pituitary tries to stimulate unresponsive ovaries. Elevated FSH with low estradiol confirms the diagnosis. The condition causes infertility, menopausal symptoms, and long-term health risks from estrogen deficiency. Hormone replacement is typically recommended.

Menopause

At menopause, FSH rises permanently as the ovaries stop producing eggs and estrogen. Elevated FSH with low estradiol confirms menopause. Testing helps distinguish menopause from other causes of missed periods in women around menopausal age.

Polycystic Ovary Syndrome (PCOS)

PCOS often shows a characteristic LH-to-FSH ratio — LH is elevated while FSH is normal or low, reversing the normal ratio. This contributes to the ovulatory dysfunction seen in PCOS. However, the LH/FSH ratio alone doesn’t diagnose PCOS.

Hypothalamic Amenorrhea

When stress, low weight, or excessive exercise suppresses the hypothalamus, FSH and LH are both low, causing periods to stop. Unlike ovarian failure, the ovaries are intact — the problem is lack of stimulation. FSH testing helps distinguish this from ovarian causes.

Male Infertility

FSH is central to evaluating male infertility. High FSH indicates testicular failure — the sperm-producing cells aren’t working despite maximal stimulation. Low FSH suggests the problem may be treatable with hormone therapy. The FSH level helps determine prognosis and treatment options.

Klinefelter Syndrome

Men with Klinefelter syndrome (XXY chromosomes) have small, dysfunctional testes that can’t produce normal sperm or testosterone. FSH is elevated because the pituitary tries to stimulate testes that can’t respond. The condition is a common genetic cause of male infertility.

Pituitary Disorders

Pituitary tumors, surgery, or other conditions can impair FSH production, causing secondary hypogonadism. Low FSH leads to understimulated gonads — absent periods and low estrogen in women, low sperm and testosterone in men. Identifying pituitary-level problems matters because they may be treatable.

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Related Biomarkers Often Tested Together

LH (Luteinizing Hormone) — Partner gonadotropin; together FSH and LH provide complete picture of pituitary-gonad axis.

Estradiol — In women, interpreted alongside FSH; low estradiol with high FSH confirms ovarian failure.

AMH (Anti-Müllerian Hormone) — Another ovarian reserve marker that doesn’t fluctuate with cycle.

Testosterone — In men, evaluated alongside FSH to assess gonadal function.

Prolactin — Elevated prolactin suppresses FSH; checked in amenorrhea evaluation.

Inhibin B — In men, reflects Sertoli cell function and provides feedback on FSH.

Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.