FSH (Follicle-Stimulating Hormone) Test

An FSH blood test measures follicle-stimulating hormone, a pituitary hormone essential to reproduction in both sexes. It is usually read together with LH and the sex hormones to evaluate fertility, the menstrual cycle, and the menopausal transition.

What the test measures

Follicle-stimulating hormone (FSH) is produced by the pituitary gland under the direction of the hypothalamus. In women, FSH stimulates the growth of ovarian follicles, each of which contains an egg, and supports estrogen production as those follicles mature. In men, FSH supports sperm production within the testes. FSH works alongside luteinizing hormone (LH), and the balance between the two reflects how the reproductive system is being regulated at any given time.

In menstruating people, FSH varies across the cycle, typically rising early to recruit a follicle and then settling as estrogen climbs. Because of this rhythm, the day of testing affects interpretation, and a result is most meaningful when matched to the correct cycle phase.

The HPG axis

FSH is part of the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases gonadotropin-releasing hormone, the pituitary responds with FSH and LH, and the ovaries or testes respond by producing eggs or sperm along with sex hormones. Those hormones feed back to the brain to adjust the next signal. When the ovaries or testes lose function, this feedback weakens and the pituitary raises FSH, which is why a high FSH often points to the gland itself rather than the brain.

Why a clinician might order it

FSH is commonly tested when evaluating fertility, irregular or absent periods, suspected menopause, or early or delayed puberty. In men it may be checked when investigating a low sperm count or low testosterone. By comparing FSH with sex hormone levels, clinicians can often distinguish a problem in the ovaries or testes from one in the pituitary or hypothalamus. It is also used to help interpret the menopausal transition alongside symptoms and age.

What happens during the test

An FSH test uses a routine blood sample drawn from a vein in the arm. The skin is cleaned, a band may be applied to make the vein easier to find, and a small tube of blood is collected in a minute or two. Mild tenderness or a small bruise afterward is common and short-lived. For people who menstruate, the draw is often scheduled for early in the cycle, and the sample is analyzed by an immunoassay in the laboratory.

How to prepare

Fasting is generally not required. For people who menstruate, the test is often timed to early in the cycle, since this is when reference ranges are best defined. Hormonal medications, including hormonal contraception and fertility treatments, can change FSH, so it helps to share a current list. A single value is interpreted in context, and a repeat draw is sometimes used, particularly during the menopausal transition when levels fluctuate.

What can affect results

• Cycle phase: FSH changes across the menstrual cycle, so the day of testing matters.
• Medications: hormonal contraception, fertility medicines, and some other drugs can raise or lower FSH.
• Life stage: FSH is low before puberty, rises through puberty, and increases as ovarian function declines around and after menopause.
• Fluctuation during the transition: in perimenopause a single value may not represent the overall trend, so repeat testing is sometimes used.

How results are generally interpreted

FSH is read together with LH and sex hormones such as estradiol or testosterone, because the pattern across these results indicates where a problem may lie:

• High FSH with low estrogen in women often reflects reduced ovarian function and is common around and after menopause.
• Low FSH with low sex hormones may suggest a pituitary or hypothalamic cause, since the pituitary is not raising its signal as expected.
• In men, high FSH can indicate that the testes are not responding well to pituitary signals, particularly affecting sperm production.
• Normal FSH with normal partner hormones is generally reassuring but is still interpreted alongside symptoms and history.

Qualitative meaning of high and low values

In broad terms, a higher-than-expected FSH suggests the ovaries or testes are providing less feedback, prompting the pituitary to signal harder, which is the expected pattern as reproductive function declines. A lower-than-expected FSH suggests reduced signaling from the pituitary or hypothalamus. Both findings gain meaning only in combination with LH, the sex hormones, age, and the clinical situation.

Reading results over time and in context

FSH is best understood as one measurement within a changing system rather than a single fixed number. In a menstruating person, matching the result to the cycle day is essential, and during the menopausal transition a repeat draw can help because levels swing from cycle to cycle. Across the lifespan, what counts as expected shifts: FSH is low in childhood, rises through puberty, follows a cyclical pattern in the reproductive years, and climbs as ovarian function declines. Interpreting any single value therefore depends on age, sex, cycle timing, and the reason for testing, which is why clinicians read FSH together with LH and the sex hormones rather than alone.

Illustrative reference ranges

The values below are illustrative only and vary by laboratory, assay, sex, age, and cycle phase. Always refer to your own report.

What the results may lead to

An FSH result usually directs the next questions rather than closing the matter. When FSH is high with low estrogen in a woman, attention often turns to ovarian function and, depending on age, to the menopausal transition or other causes of reduced ovarian activity. When FSH is low with low sex hormones, the focus shifts toward the pituitary and hypothalamus, and further testing or imaging may follow. In men, a high FSH may prompt a closer look at sperm production and testicular function alongside testosterone. In fertility care, FSH is commonly combined with LH, the sex hormones, and cycle information to form a fuller assessment.

Because FSH is one part of a connected system that changes with the cycle and across life, a single value is interpreted with care. The same number can mean different things depending on cycle day, age, sex, and the reason for testing, so a result that appears unusual in isolation may be expected once context is known. A clinician may repeat the test, time it to the cycle, or pair it with other measurements before reaching conclusions, and may also weigh non-hormonal explanations for the symptoms involved.

Related tests

FSH is almost always interpreted with the LH test and with the sex hormones it helps regulate. When reproductive symptoms are present, prolactin may be added, since elevated prolactin can disturb the cycle, and thyroid tests such as the free T4 or free T3 may be considered when thyroid involvement is suspected. See the blood tests index, the hormones library, or read about related conditions.