How to prepare for an FSH blood test
Follicle-stimulating hormone is a cornerstone of fertility and hormone assessment in both women and men, but a poorly timed sample can send you down the wrong path. Our editorial team sets out how to prepare so that your result is genuinely useful.
What FSH is and why it is measured
Follicle-stimulating hormone (FSH) is produced by the pituitary gland at the base of the brain. In women, it drives the growth of ovarian follicles, each containing an egg, and stimulates oestrogen production. In men, it acts on the Sertoli cells of the testes to support sperm production.
FSH is controlled by a feedback loop. The hypothalamus releases gonadotrophin-releasing hormone (GnRH), which prompts the pituitary to secrete FSH and its partner luteinising hormone (LH). Sex hormones and a protein called inhibin then signal back to slow production when levels are sufficient.
Doctors request FSH for several reasons: to investigate fertility problems, irregular or absent periods, suspected early menopause, delayed puberty, low testosterone in men and pituitary disorders. In athletes, it is also used to assess whether the reproductive axis has been suppressed by low energy availability or by exogenous hormones.
In the UK, results are reported in international units per litre (IU/L). Because "normal" depends heavily on sex, age and, for women, the phase of the menstrual cycle, preparation and timing make a real difference to whether the result can be interpreted at all.
Timing the test
For women with a regular cycle, FSH is usually measured early in the cycle, typically between days two and five, counting the first day of full bleeding as day one. This is the point at which hormone levels are relatively stable and comparable with reference ranges. A test taken mid-cycle can show a natural surge that looks alarming but is entirely expected.
Women with irregular or absent periods may be tested on any day, but the clinician will interpret the result alongside other hormones such as LH, oestradiol and prolactin. Guidance on polycystic ovary syndrome, such as the international evidence-based guideline led by Teede and colleagues, outlines how these tests fit into diagnosis.
Men can usually be tested on any day. Many clinicians prefer a morning sample, especially when FSH is checked alongside testosterone, which is highest in the early morning. NICE guidance on fertility assessment and the Endocrine Society guideline on testosterone therapy both reflect this approach.
For suspected menopause, NICE guidance (NG23) advises that FSH testing is not needed to diagnose menopause in women over 45 with typical symptoms. It may be used in younger women when premature ovarian insufficiency is suspected, often repeated on two occasions several weeks apart.
Things that can distort the result
Several everyday factors can make an FSH result misleading. The most important is hormonal medication. Combined contraceptive pills, patches and rings suppress FSH, as do some hormonal coils to a lesser degree. Hormone replacement therapy changes the picture too.
| Factor | Likely effect on FSH | What to do |
|---|---|---|
| Combined hormonal contraception | Suppresses levels | Tell the clinician; they may advise a break before testing |
| Testosterone or anabolic steroids | Strongly suppresses levels | Disclose use honestly |
| Low energy availability | Can suppress the axis | Mention dieting, weight cuts or heavy training |
| High-dose biotin supplements | Can interfere with some lab assays | Ask whether to pause beforehand |
| Acute illness | Temporary changes | Postpone routine tests if unwell |
Biotin deserves special mention because it is common in hair, skin and nail supplements and in some sports products. At high doses, it can interfere with immunoassays used for several hormones, producing falsely high or low readings depending on the test design. Regulators have issued warnings on this, and many laboratories advise stopping high-dose biotin for a period before testing.
Exogenous testosterone and anabolic steroids switch off the pituitary's output of FSH and LH through negative feedback. Rahnema and colleagues describe how this suppression underlies steroid-induced hypogonadism and infertility, which can persist after use stops.
Practical checklist before the appointment
A few simple steps help ensure your sample is useful. Your GP surgery or the laboratory may give specific instructions, and those should take priority.
- Confirm the day: women with regular cycles should usually book for days two to five.
- Book a morning appointment if testosterone or other hormones are being tested at the same time.
- Ask whether you need to fast; FSH alone usually does not require fasting, but combined panels might.
- List all medicines and supplements, including contraception, biotin and any hormones.
- Avoid an unusually hard training session or competition the day before.
- Postpone the test if you are acutely unwell.
Drink water as normal, as dehydration can make blood draws harder. There is no need for special diets or avoiding caffeine for FSH specifically, unless you have been instructed otherwise.
Bring a note of the first day of your last period and of any recent changes in cycle length. This information helps the clinician interpret results more accurately.
Understanding the results
FSH is rarely interpreted in isolation. In women, a high FSH with low oestradiol may point to reduced ovarian reserve or menopause; low FSH and LH with low oestradiol may suggest hypothalamic suppression, which is common in athletes with low energy availability. Burger and colleagues have described the gradual rise in FSH that accompanies the transition to menopause.
In men, high FSH with a low sperm count suggests a problem in the testes themselves, while low FSH with low testosterone suggests the signal from the pituitary is lacking. The latter pattern is typical after anabolic steroid use.
Because FSH can vary from cycle to cycle, a single abnormal result is often repeated. Do not make decisions about fertility or treatment based on one reading.
Our editorial team does not provide guidance on medicines used to manipulate FSH, such as those used in fertility treatment or post-steroid recovery. These require specialist supervision, and self-treatment can cause harm.
Editorial conclusions
FSH is a key reproductive hormone, and its interpretation depends on timing, sex, age and context. For women with regular cycles, testing on days two to five is standard.
Hormonal contraception, anabolic steroids, low energy availability and high-dose biotin can all distort results, so honesty about medicines and supplements is essential.
Results are interpreted alongside LH, oestradiol, testosterone and clinical history, and abnormal values are usually repeated.
Related articles you may find useful cover LH testing, testosterone blood tests for men, and the hormonal effects of low energy availability.
References
- National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. Clinical guideline CG156. London: NICE; 2013.
- National Institute for Health and Care Excellence. Menopause: diagnosis and management. NICE guideline NG23. London: NICE; 2015.
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744.
- Teede HJ, Tay CT, Laven JJE, et al. Recommendations from the 2023 international evidence-based guideline for the assessment and management of polycystic ovary syndrome. J Clin Endocrinol Metab. 2023;108(10):2447–2469.
- Rahnema CD, Lipshultz LI, Crosnoe LE, et al. Anabolic steroid-induced hypogonadism: diagnosis and treatment. Fertil Steril. 2014;101(5):1271–1279.
- Burger HG, Dudley EC, Robertson DM, Dennerstein L. Hormonal changes in the menopause transition. Recent Prog Horm Res. 2002;57:257–275.
