What is FSH?

Follicle-stimulating hormone (FSH) is responsible for many different aspects of health, from sperm production to regulating the menstrual cycle. Therefore, abnormal levels can give rise to all sorts of sexual and reproductive symptoms. 

This guide covers the role of FSH, normal levels for your age, and what happens if your levels are abnormal.

We cover:

• What is FSH?
• Why might I need an FSH test?
• Normal range for FSH
• Causes of raised FSH levels
• Causes of low FSH levels
• Normal FSH for pregnancy
• LH:FSH ratio
   

What is FSH?

FSH is a hormone produced by the pituitary gland. It is one of the gonadotrophic hormones, alongside luteinising hormone (LH). Gonadotrophic hormones stimulate the growth and activity of the gonads (testes and ovaries).
 

What does FSH do?

As its name suggests, one of the primary roles of follicle-stimulating hormone is to stimulate the growth of the follicles. Follicles are the small sacs released from the ovaries in which the eggs grow. However, FSH has many other important functions. 

FSH also: 

• Helps the follicles make oestrogen
• Regulates the menstrual cycle
• Promotes the production and maturation of sperm in the testes
   

Why might I need an FSH test?

There are several reasons you might benefit from an FSH blood test. Often, it’s to investigate reproductive health. 

An FSH blood test may be helpful to:

• Find out why periods have become irregular
• Investigate fertility issues
• Monitor and manage fertility treatments
• Help confirm menopause if the diagnosis is unclear
• Check if the testicles are working normally
• Investigate causes of testosterone deficiency (hypogonadism)
• Investigate conditions related to the hypothalamus or pituitary gland
   

How can I check my FSH levels?

You can check your levels from home with an FSH blood test. We recommend one of the following tests, which all measure your FSH levels:

• Female Hormone Blood Test
• Male Hormone Blood Test
• Advanced Well Woman Blood Test
• Advanced Well Man Blood Test
• Ultimate Performance Blood Test

The first two options allow you to take your sample yourself as a finger-prick test. 
 

What is the normal range for FSH by age?

FSH levels vary depending on age, sex, and the stage of the menstrual cycle. Ideally, an FSH test is taken on day three of the menstrual cycle, or at least between days two and five. If you take your test at a different time, your result could be higher or lower than expected.
 

What causes a raised FSH result?

Higher FSH levels are often a sign of a condition affecting the ovaries or testes. If they can’t make enough sex hormones, the pituitary gland in the brain responds by releasing more FSH. 

Raised FSH levels may be due to: 

In women

• Low ovarian reserve or primary ovarian insufficiency (POI)
• Menopause or perimenopause
• Polycystic ovary syndrome (PCOS)
• Thyroid or adrenal gland conditions
• Tumour of the ovary or pituitary gland (rare)
• Turner’s syndrome (a chromosomal abnormality)

In men

• Injury to the testes
• Klinefelter’s syndrome (chromosomal abnormality)
• Mumps
• Previous chemo- or radiotherapy to the testes
• Stress (which can lower testosterone and cause a secondary rise in FSH)
• Tumour of the germ cells (the cells that produce sperm)

FSH may be falsely raised in people with heterophilic antibodies – these are human antibodies against animal antibodies. They’re thought to arise from exposure to animals or animal serum products. Although fairly common in the general population, only up to one in 200 samples seem to be affected by this phenomenon [1]. 
 

What causes a low FSH result?

Low FSH levels can be caused by the pituitary gland not releasing as much FSH as it should.

Low FSH levels may be due to:

• Anovulation (not ovulating)
• Chronic stress
• Gonadotrophin-releasing hormone (GnRH) agonists (long-term use), often used to treat conditions like endometriosis, fibroids, and some cancers
• Hormonal contraceptives
• Hypopituitarism (when the pituitary gland doesn’t produce enough hormones)
• Rapid weight loss or being very underweight (often accompanied by a change in periods)
• Pregnancy
   

What is a normal FSH result to get pregnant?

Many factors affect fertility and your FSH level is just one of them. Generally, an FSH level of less than 10 IU/L, taken on day three of your cycle, is a normal result if you’re looking to conceive. A higher result doesn’t mean you won’t be able to get pregnant, but it might mean your chances are lower. 

Women who struggle to conceive tend to have slightly higher levels of FSH. For example, one study showed that infertile women’s FSH levels were on average 10 to 20 IU/L higher than fertile women. A better marker of ovarian reserve is AMH, which you can check with an Anti-Müllerian Hormone (AMH) Blood Test. 
 

What happens to FSH during perimenopause and menopause?

FSH levels start to gradually increase about six years before menopause. During this period, levels can fluctuate significantly, so you may notice your levels rising and falling. There is a sharper increase in the two years preceding menopause. After menopause, levels rise for another few years before stabilising [2].

If you’re menopausal, your FSH levels will likely be over 25.8 IU/L. Usually, menopause is diagnosed based on your symptoms and a blood test isn’t necessary. In some circumstances, measuring FSH may be useful, especially where symptoms aren’t typical or menopause is thought to be happening early (before the age of 45) [3]. 
 

What is the LH:FSH ratio?

Normally, luteinising hormone (LH) and FSH levels are similar, giving an LH:FSH ratio of 1 or less.

With polycystic ovary syndrome (PCOS), often the LH:FSH ratio is raised – this occurs in about 60% of women with the condition [4]. However, this ratio is not diagnostic.

Find out more about how PCOS is diagnosed.
 

LH:FSH ratio calculator

Enter your LH level: Enter your FSH level: Calculate ratio

What should I do if my FSH levels are abnormal?

Depending on your results and the extent of your symptoms, it may be appropriate to retest in a few months or visit your GP for further follow-up. 

Our doctors will provide you with appropriate next steps on what to do if your result is abnormal. 
 

References

1. Ismail AA, Barth JH. Wrong biochemistry results. BMJ. 2001;323: 705–706. doi:10.1136/bmj.323.7315.705
2. Mao L, Wang L, Bennett S, Xu J, Zou J. Effects of follicle-stimulating hormone on fat metabolism and cognitive impairment in women during menopause. Front Physiol. 2022;13: 1043237. doi:10.3389/fphys.2022.1043237
3. Diagnosis of menopause and perimenopause | Diagnosis | Menopause | CKS | NICE. [cited 19 Apr 2024]. Available: https://cks.nice.org.uk/topics/menopause/diagnosis/diagnosis-of-menopause-perimenopause/
4. Xia Q, Xie L, Wu Q, Cong J, Ma H, Li J, et al. Elevated baseline LH/FSH ratio is associated with poor ovulatory response but better clinical pregnancy and live birth in Chinese women with PCOS after ovulation induction. Heliyon. 2023;9: e13024. doi:10.1016/j.heliyon.2023.e13024