Follicle Stimulating Hormone (FSH)

FSH at a glance

What is Follicle Stimulating Hormone?

Follicle stimulating hormone, or FSH, is a hormone that plays a central role in reproduction and sexual development in both women and men. It belongs to a family of hormones called gonadotrophins, which act on the gonads: the ovaries in women and the testes in men.

In women, FSH does exactly what its name suggests: it stimulates the ovarian follicles, the tiny fluid-filled sacs that each contain an immature egg. At the start of every menstrual cycle, FSH prompts a group of these follicles to begin growing and maturing. In men, FSH supports the production of sperm.

Because it sits at the heart of fertility and the reproductive system, FSH is one of the hormones most often measured when someone is investigating fertility, irregular periods, or the menopause.

Where is FSH produced?

FSH is made and released by the pituitary gland, a pea-sized gland at the base of the brain. It takes its instructions from the hypothalamus, which releases Gonadotrophin-Releasing Hormone (GnRH). GnRH tells the pituitary when to release FSH — and Luteinising Hormone (LH) — into the bloodstream. From there, FSH travels to the ovaries or testes to do its work.

This chain (hypothalamus to pituitary to ovaries or testes) is the hypothalamic-pituitary-gonadal (HPG) axis. It works as a feedback loop: when the ovaries or testes produce enough of their own hormones, they signal back to the brain to ease off FSH production. This is why FSH levels rise when the ovaries stop responding, as happens during the menopause.

Role of Follicle Stimulating Hormone

FSH is active at almost every stage of life, from before birth through to old age. Its job changes depending on when and in whom it is working.

FSH role in foetal development

FSH is involved in reproductive development before birth. In the developing foetus, the pituitary gland begins producing FSH partway through pregnancy, contributing to the early formation of the ovaries and testes. In female foetuses in particular, FSH helps establish the pool of ovarian follicles that a woman is born with and will draw on throughout her reproductive life [1].

FSH role during puberty

Puberty begins when the hypothalamus increases its release of GnRH, driving the pituitary to produce more FSH and LH. In girls, rising FSH stimulates the ovaries to begin maturing follicles and producing oestrogen, setting the menstrual cycle in motion. In boys, FSH stimulates the testes and supports the start of sperm production.

FSH role in the menstrual cycle

In women of reproductive age, FSH drives the early phase of each menstrual cycle. In the follicular phase, FSH rises and prompts a cohort of follicles to grow; usually one becomes dominant. As it matures it produces oestrogen, which feeds back to the brain and causes FSH to dip. A surge in LH around mid-cycle then triggers ovulation. After ovulation the ruptured follicle becomes the corpus luteum, producing progesterone and keeping FSH low through the luteal phase [2]. This is why FSH is not fixed and why the day a sample is taken matters so much.

FSH role in males

In men, FSH levels are steady rather than cyclical. FSH acts on the Sertoli cells in the testes to support sperm production [3]. Alongside testosterone, it is essential for male fertility. Because men have no monthly cycle, a man’s FSH can be measured on any day and is generally more stable than a woman’s.

FSH and LH: why these hormones are looked at together

FSH is almost always considered alongside luteinising hormone (LH). The two are produced by the same gland, released in response to the same signal, and work as a pair.

Early in the cycle, FSH and LH are usually present in roughly equal amounts (about 1:1). A lower ratio can accompany a reduced ovarian reserve; a higher ratio (LH around twice FSH or more) is one pattern sometimes seen in Polyendocrine Metabolic Ovarian Syndrome (PMOS): here FSH itself is usually normal or on the low side, and it’s the raised LH that tips the ratio.

Like FSH, LH stays relatively stable through the reproductive years and rises after the menopause [5]. In a post-menopausal woman both are typically high, and it is the combination, read with symptoms and cycle history, that tells the fuller story.

Normal FSH Levels

There is no single normal FSH level that applies to everyone. What counts as normal depends on your sex, your age, your cycle phase, whether you have reached the menopause, and whether you are taking HRT or hormonal contraception.

Forth FSH reference ranges

Reference ranges supplied by Forth’s NHS laboratory partner (Royal Exeter).

An important point: the same FSH number can be Low, Healthy or High depending on which range applies. A reading of 30 IU/L is well above range for a woman in the first half of her cycle, but entirely normal after the menopause.

In practice, the table means that for women who are still having periods FSH usually sits within the follicular-phase range of roughly 3.5–12.5 IU/L (Forth’s pre-menopausal median is around 6.8 IU/L), while for men it stays fairly steady between about 1.5 and 12.4 IU/L at any adult age. 

One practical difference follows directly: because a man’s FSH is stable rather than cyclical, a single well-timed test gives a reliable picture, whereas a woman’s result depends heavily on where she is in her cycle and on her life stage 

Normal FSH levels by age

Across Forth results (excluding anyone on HRT or hormonal contraception) the pattern by age is striking and completely different for women and men. For women, FSH stays flat through the twenties and thirties, edges up in the early forties, then climbs sharply from the mid-forties. For men, it rises only gently across adult life.

FSH levels by phase of the menstrual cycle

Because FSH opens and closes the cycle, its level depends on which day a sample is taken. Forth results show FSH is highest in the early follicular phase and lowest in the luteal phase, which is why FSH is usually measured around days 2–5.

How often is FSH Low, Healthy or High?

The proportion of women whose FSH falls outside the healthy range climbs with age: from around 3% at 18–24, to roughly 13% at 35–39, 22% at 40–44, 40% at 45–49 and 48% at 50–54.

A note on reading this: once a woman is post-menopausal, her result is judged against the higher post-menopausal range (25–135 IU/L), so a very high FSH is then classified as normal for that life stage. This is why the proportion flagged high falls again in the late fifties, not because FSH drops, but because the healthy range moves with you.

For men, raised FSH is uncommon in early and mid-adulthood (under about 4%), rising more noticeably after 55 [4].

FSH levels and fertility

FSH is one of the first hormones looked at in fertility investigations, because it offers a window onto how the ovaries are responding. But it is only one part of the overall picture.

FSH describes ovarian response, not your chance of conceiving. It is read alongside everything else.

What is a normal FSH level to get pregnant?

Measured on days 2–5, a level within the follicular range (often quoted as ideally under about 10 IU/L) is generally reassuring. Across Forth results from women tested in this window (no HRT or contraception), around 83% had an FSH in the healthy range, with about 13% raised and 3% low. But the share with a raised early-cycle FSH climbs from under 2% in the twenties to about 8% at 35–39, 18% at 40–44 and 36% at 45–49.

What FSH can suggest about ovarian response

A well-timed FSH result is essentially a measure of how hard the brain is working to stimulate the ovaries. As ovarian reserve declines with age, the brain produces more FSH, so a rising early-cycle level can be an early sign of diminishing ovarian response, often before periods change.

Why FSH cannot predict pregnancy on its own

FSH describes ovarian response, not your ability to conceive. It cannot count your remaining eggs, ignores your partner’s fertility, and varies cycle to cycle. It is read together with age, other hormones such as Anti-Müllerian Hormone (AMH), LH and oestradiol, ovulation, medical history and partner factors.

FSH in perimenopause, menopause and postmenopause

As the ovaries wind down, FSH becomes one of the clearest hormonal signals of the transition but also one of the most variable.

Forth results, women, HRT and contraception excluded. The transition is a widening cloud, not a line.

Why FSH fluctuates during perimenopause

Around the menopause the ovaries sputter rather than shut down smoothly, and FSH rises and falls accordingly. In Forth’s 45–49 bracket, results range from under 3 IU/L to over 100 IU/L. A single reading can look normal one month and high the next in the same woman. The variability is the perimenopause.

Can you be perimenopausal with a normal FSH?

Yes. Because FSH swings so much, a single normal result does not rule out perimenopause. A test taken on a good ovarian month can come back reassuringly normal even when the transition is well underway.

What FSH level may indicate menopause?

Menopause is usually diagnosed clinically after 12 consecutive months without a period. A persistently high FSH (typically above about 25–30 IU/L on more than one occasion) with absent periods is consistent with it.

Typical FSH ranges by life stage

Bringing it together, FSH broadly follows an arc across a woman’s life. The key shift comes at the end of it: after the menopause FSH settles at a consistently high level and stays there, so a result that would be strikingly high in a younger woman is simply the new normal once the ovaries are no longer active [6].

HRT changes this picture. In post-menopausal women taking HRT, Forth’s median FSH is around 41 IU/L (lower than the ~73 IU/L seen without HRT) because HRT partly restores the feedback signal. This is one reason FSH is a poor way to judge menopause status in anyone on HRT.

Low FSH levels: causes, symptoms and what they may mean

What causes low FSH?

A low FSH means the pituitary is not releasing much of it. Possible reasons include:

• Problems with the hypothalamus, such as Kallmann syndrome, an inherited condition that delays or prevents puberty and also reduces or removes the sense of smell.
• Pituitary disorders, including prolactinomas (benign pituitary tumours), hypopituitarism, and Sheehan syndrome (pituitary damage after heavy blood loss in childbirth) [7].
• Medications and treatments, including hormonal contraception [8], testosterone replacement therapy in men [9], chemotherapy [10], and long-term GnRH agonists or antagonists.
• Pregnancy, when high oestrogen and progesterone keep FSH suppressed [11].
• An underactive thyroid (hypothyroidism), which can lower both FSH and LH [12].
• Significant stress, very low body weight, or excessive exercise, all of which can suppress the brain’s reproductive signalling. This includes Relative Energy Deficiency in Sport (RED-S), where a sustained energy shortfall lowers FSH in athletes of either sex.
• PMOS, in which FSH is typically normal or low-normal while LH is elevated, resulting in a raised LH:FSH ratio rather than a genuinely low FSH [16].

Symptoms of low FSH levels

Low FSH often has no direct symptoms; effects come from the low oestrogen or testosterone that can follow. 

These may include:

• irregular or absent periods 
• difficulty conceiving
• low libido
• reduced sperm production 
• erectile difficulties
• weight gain or loss [13].

High FSH levels: causes, symptoms and what they may mean

A high FSH is the more common abnormal result, usually reflecting the brain working harder to stimulate ovaries or testes that are becoming less responsive.

What causes high FSH levels?

Raised FSH generally means the pituitary is pushing hard for lack of feedback from the gonads. Common causes include:

• Menopause and the approach to it: by far the most common cause in women.
• Premature ovarian insufficiency (premature ovarian failure), when the ovaries stop working before 40
• Klinefelter syndrome in men, a genetic condition (an extra X chromosome) that impairs testicular function [14].
• Turner syndrome in women, a genetic condition where an X chromosome is missing or partial [15].

In men, raised FSH is uncommon when young but becomes more frequent with age: from under 2% in early adulthood to around 13% past 60. It can indicate that the testes are not responding fully, which is often relevant when investigating fertility or low testosterone, and usually warrants a conversation with a doctor.

In women, the likelihood of a raised FSH rises steeply with age, tracking the menopausal transition: the proportion with a raised result climbs from a few per cent in early adulthood to around 40–48% in the late forties and early fifties. A high result in a younger woman is more likely to prompt investigation, whereas in a woman in her late forties it is often an expected part of the transition.

Can FSH be high if you still have regular periods?

Yes, this catches many people out. FSH often starts rising before periods become irregular. Among Forth women with a high FSH, the majority in the range 18-40 still reported regular periods:

Symptoms of high FSH levels

High FSH itself does not cause symptoms directly; symptoms come from the low oestrogen or testosterone alongside it, and reflect the underlying condition.

How to check and track your FSH levels

What is an FSH blood test?

An FSH blood test measures the amount of FSH in your blood, in international units per litre (IU/L). It is often done as part of a wider hormone panel, on a venous sample or, with Forth, a finger-prick sample taken at home.

When should an FSH test be done?

For women still cycling, FSH is usually measured in the early follicular phase, on days 2–5 (day 1 is the first day of your period). Men can be tested on any day. 

Hormonal contraception and HRT both alter FSH, so results during treatment need to be interpreted with that in mind.

How to understand your FSH blood test results

Your result comes with a reference range and a label (Low, Healthy or High) based on your sex and, where relevant, cycle phase or menopause status. The most important thing is context: the same number means different things at different life stages.

Anatomy of an FSH result:

• Result & units. The measured value, in international units per litre (the standard for FSH).
• Reference range. Which range applies depends on sex and cycle phase. Here the follicular range, because the sample was day 3.
• Label. Low, Healthy or High: where the value sits within that range.
• Your context. Sex, age, cycle day and collection type all change how the same number should be read.

What your FSH result can and cannot tell you

FSH is useful: it can flag reduced ovarian response, support a menopause picture, and help investigate fertility. But it varies day to day and cycle to cycle, especially in perimenopause, so one reading can mislead. 

Contraception and HRT change it: the effect of contraception is modest in younger women but marked around the menopause, where the pill can mask the natural rise.

Forth results, women. Contraception samples are small at older ages. 

Are over-the-counter menopause tests accurate?

Some over-the-counter menopause tests measure FSH in urine, like a pregnancy test. Because they check one hormone on one day, they cannot reliably tell whether you are in perimenopause or menopause. FSH varies too much, and the RCOG recommends against relying on them [17]. A blood test that maps several hormones across the cycle gives a far more accurate picture.

Forth tests that include FSH

FSH is included in several Forth hormone and fertility tests, letting you check your level from home and track how it changes over time. Choose the one that fits what you want to understand.

• Female Hormone Imbalance Blood Test: a single-day snapshot of your key female hormones (including FSH, LH, oestrogen and progesterone) alongside thyroid function. 11 biomarkers, and suitable if you use hormonal contraception.
• MyFORM® Female Hormone Mapping Blood Test tracks your hormones across the whole menstrual cycle to show how well they are working. Not suitable if you are taking hormonal contraception.
• MyFORM® Perimenopause Blood Test maps your hormones across your cycle to give a clearer read on whether you are in perimenopause than a single-day test can.
• Female Fertility Blood Test: for anyone trying to conceive, or struggling to, it gives a fuller picture of the hormones that shape your reproductive health.

FAQs about FSH