FSH (Follicle-Stimulating Hormone) Blood Test

What Does This Test Measure?

Follicle-stimulating hormone is released by the pituitary gland and acts on the gonads. In women it stimulates ovarian follicles to grow and mature each month; in men it drives sperm production in the testes. It works in tandem with luteinising hormone, and the two are almost always measured together.

FSH sits inside a feedback loop. The gonads produce estradiol, testosterone and inhibin, which suppress FSH release. When the gonads fail, that suppression is lost and FSH climbs, which is why a high FSH is a marker of gonadal failure rather than of pituitary disease.

The direction of the abnormality is therefore the key piece of information. A high FSH means the pituitary is working and the gonad is not. A low FSH alongside low sex hormones means the pituitary or hypothalamus is the problem.

Why Your Doctor Ordered This Test

In women, FSH is measured for irregular or absent periods, infertility, suspected premature ovarian insufficiency, and to support a diagnosis of menopause. A day-3 FSH has long been used as a rough indicator of ovarian reserve, though anti-Müllerian hormone has largely superseded it for that purpose.

In men, it is checked for infertility or a low sperm count, small testes, or delayed puberty. In both sexes it forms part of the assessment of suspected pituitary disease, and in children it is used to investigate early or delayed puberty.

What High FSH Means

A high FSH means the gonads are not producing enough hormone to suppress it. In women this is the defining feature of menopause, and when it occurs before 40 alongside a low estradiol it indicates premature ovarian insufficiency, which warrants investigation for autoimmune causes and chromosomal conditions such as Turner syndrome.

In men, a high FSH with a low sperm count indicates primary testicular failure; the testes are not responding. Causes include Klinefelter syndrome, previous mumps orchitis, chemotherapy or radiotherapy, undescended testes, and testicular injury. A high FSH in this setting generally predicts that sperm production will not respond to hormonal treatment.

Rarely, a raised FSH with raised sex hormones indicates a pituitary tumour producing FSH directly, or resistance to hormone feedback.

What Low FSH Means

A low FSH alongside low estradiol or low testosterone indicates hypogonadotropic hypogonadism; the signal from above is missing. The commonest explanations are functional rather than structural: significant weight loss, low body fat, eating disorders, heavy endurance training, chronic illness and severe stress all suppress the axis, and it usually recovers when the underlying situation resolves.

Structural causes include pituitary tumours, particularly prolactin-secreting ones, along with pituitary surgery, radiotherapy, head injury and infiltrative disease. Anabolic steroid use and testosterone therapy suppress FSH strongly and are a frequent and often undeclared cause of a low result with impaired fertility in men. Opioids and high-dose corticosteroids do the same.

How to Prepare and What Affects the Result

Fasting is not needed. For women, the cycle day is essential and should be recorded; baseline samples are normally taken on day 2 to 5. For men, timing is not critical, though a morning sample is conventional when testosterone is measured alongside.

Hormonal contraception suppresses FSH and generally needs to be stopped for a period before meaningful testing. Testosterone therapy, anabolic steroids, opioids and corticosteroids all lower results. Biotin supplements interfere with many immunoassays. Because FSH is released in pulses, a single value can be misleading and repeat sampling is sometimes required.

Common Misinterpretations

The most common error is drawing conclusions from FSH without LH and the relevant sex hormone. The three together locate the problem; FSH alone rarely does.

The second is using a single raised FSH to diagnose menopause during the perimenopause, when levels fluctuate widely from cycle to cycle. A raised value can be followed by an ovulatory cycle, and menopause remains primarily a clinical diagnosis.

The third is over-reading a day-3 FSH as a fertility verdict. It is a crude measure of ovarian reserve, insensitive to early decline, and considerably less informative than anti-Müllerian hormone and antral follicle count. A normal FSH does not confirm good reserve.

Frequently Asked Questions

What does a high FSH mean for fertility?

In women it suggests the ovaries are responding less well to stimulation, and a persistently raised day-3 value is associated with reduced ovarian reserve. In men it usually indicates the testes themselves are not producing sperm effectively. In both cases it points to a gonadal rather than pituitary problem, but it is not an absolute verdict and should be interpreted alongside other tests.

Can FSH confirm I have gone through menopause?

It supports the diagnosis but does not make it alone. During the perimenopause FSH swings widely, and a high value can be followed by a normal ovulatory cycle. Menopause is diagnosed clinically after twelve consecutive months without periods, with a persistently raised FSH and low estradiol used as supporting evidence, particularly in younger women.

Why are FSH and LH nearly always ordered together?

Because the pattern between them is more informative than either alone. Both raised indicates gonadal failure. Both low with low sex hormones indicates a pituitary or hypothalamic cause. A raised LH with a normal FSH and high testosterone in women suggests polycystic ovary syndrome. The combination narrows the diagnosis in a way one hormone cannot.

Does testosterone therapy affect FSH?

Strongly. External testosterone suppresses the pituitary, driving FSH and LH down and shutting off natural sperm production. This is a common cause of infertility in men on testosterone or anabolic steroids, and it is frequently not declared. Recovery after stopping can take many months and is not always complete.

Why does the cycle day matter for women?

Because FSH varies across the cycle, rising in the early follicular phase, surging at mid-cycle and falling in the luteal phase. Baseline assessment is standardised to day 2 to 5 so that results are comparable. A sample taken at an unrecorded point in the cycle is often uninterpretable.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your FSH result: