What is the follicle-stimulating hormone (FSH)?

What does FSH do?

FSH initiates the production of oestradiol (E2) from the follicles in your ovaries. FSH and E2 work together to grow and develop these follicles containing eggs. Mid-cycle, the follicle ruptures and the egg is released — ovulation. After ovulation, FSH levels remain low throughout the luteal phase (around day 15 of a 28-day cycle), preventing the development of new follicles.  

For women undergoing in vitro fertilisation (IVF), FSH is given as an injection to help stimulate follicles so that multiple eggs can be harvested and collected for fertilisation. 

FSH also helps produce other essential sex hormones — like oestrogen and testosterone. 

Is there an FSH blood test? 

You can check your FSH levels as part of a women’s hormones blood test. It’s measured along with other health markers — like testosterone, oestrogen, sex hormone-binding globulin (SHBG), and luteinising hormone (LH).

An FSH test can be helpful for both men and women if you’re investigating fertility problems. It can help uncover the causes of irregular menstrual cycles and diagnose certain disorders in women. 

In men, an FSH test can help identify the reason for low sperm count or problems with the testicles. 

What’s the normal range for FSH?

FSH levels vary depending on your sex assigned at birth and life stage. 

One common range for premenopausal women is 3.5 - 12.5 IU/L. We recommend testing FSH levels on day 3 of your cycle — when they’re most accurate.

What causes high FSH levels?

There are many causes for high FSH levels. These include:

• Stress — when your cortisol levels become too high, this can lead to other hormonal imbalances.

• Polycystic ovary syndrome (PCOS) — you might have an elevated LH:FSH ratio if you have PCOS. This hormone dysregulation can be the reason for irregular ovulation. 

• Early menopause — you stop having periods before age 45. High FSH levels occur alongside a lack of periods. 

• Turner syndrome — a genetic condition in females when the second X chromosome is missing or partially missing. This can lead to underdeveloped ovaries, irregular periods, and infertility. FSH levels above the normal range (before adolescence) can help indicate this condition. 

• Klinefelter syndrome  — a genetic condition where males are born with an extra X chromosome (47XXY). This can lead to infertility and low sex drive (libido). 

• Menopause — your FSH levels become naturally higher due to lower oestrogen levels signalling your brain to produce more FSH to stimulate more follicles. There aren't enough follicles to use the extra FSH, so FSH levels rise. Most women don’t need a blood test to diagnose menopause — your hormones fluctuate during the first few years, so you won’t get a clear picture of your FSH levels.

What causes low FSH levels?

Low FSH levels can sometimes indicate certain health conditions. It’s also normal to have low FSH levels in pregnancy. 

If you have low FSH levels, you might have trouble getting pregnant because you’re not ovulating. And in men, it can signify a low sperm count. You might consider checking your FSH levels if you're having trouble getting pregnant. 

Low FSH levels can also indicate a problem with the hypothalamus (the part of your brain responsible for keeping your body stable) or the pituitary gland. 

Other problems with your pituitary gland or hypothalamus and subsequent low FSH levels can lead to irregular periods and a lack of ovulation, impacting fertility. 

Certain lifestyle factors might cause this to happen, like: 

• low body weight
• low-calorie intake
• sustained excessive exercise 
• rapid weight loss

How to support your FSH levels

The good news is you can reencourage FSH production by:

• Reducing your stress levels — try relaxing activities like yoga or meditation and prioritise your sleep. 
• Avoiding excessive exercise — aim for a mix of activity types (like strength training and aerobic activity) and consider no more than 75 minutes of vigorous-intensity activity or 150 minutes of moderate-intensity activity a week.
• Eating regular meals — aim for three meals a day and three healthy snacks.
• Quit smoking — smoking can raise your FSH levels.
• Reduce your alcohol intake — moderate or high alcohol intake can lead to high FSH levels.
  

If you’re worried about your body image, your GP can support you to bring this to a healthy weight. You might also consider seeing a professional therapist — they can help you discuss emotional factors contributing to your weight and body image.