How to prepare for the ‘FSH’ test: rules for taking it

Follicle-stimulating hormone (FSH) is a pituitary hormone on which the maturation of follicles in the ovaries and the production of spermatozoa in the testes depend. The FSH test is used to assess ovarian reserve, diagnose menopause, male infertility and hypogonadism. For the result to be informative, it is important to choose the right day, to know which drugs alter it, and to take it together with the necessary indicators.

The role of FSH in the body

FSH, like LH, is produced by the gonadotropic cells of the pituitary under the control of gonadotropin-releasing hormone. In women, FSH stimulates the growth of follicles and the synthesis of estradiol by granulosa cells. In men — it acts on the Sertoli cells in the testes, which ensure spermatogenesis.

The regulation of FSH has its own peculiarity: in addition to sex steroids, its secretion is inhibited by inhibin B — a protein produced by the ovarian follicles and Sertoli cells. Therefore FSH well reflects the ‘state of affairs’ precisely in the germinal part of the sex glands.

Compared with LH, FSH secretion is less markedly pulsatile, so a single result is a little more stable. However, in women the FSH level changes noticeably over the cycle, which determines the rules for the draw.

The test is ordered in infertility of both partners, menstrual cycle disorders, suspected premature ovarian insufficiency, in perimenopause, in delayed sexual development, and in men — with an altered spermogram, small testicular volume and low testosterone.

For women: the day of the cycle and the ‘pair’ with estradiol

Baseline FSH for assessing ovarian function is measured on day 2–5 of the menstrual cycle. In this period the follicles have not yet begun to grow actively, estradiol is low, and FSH reflects how ‘strenuously’ the pituitary is stimulating the ovaries.

It is advisable to measure estradiol in the same draw. If in the early follicular phase estradiol is elevated, it suppresses FSH through feedback, and a normal FSH figure may ‘mask’ a reduced ovarian reserve. Without estradiol such a result is difficult to interpret.

The Practice Committee of the American Society for Reproductive Medicine (ASRM), in its conclusion on ovarian reserve tests, notes that basal FSH has significant variability between cycles and limited sensitivity. Therefore the modern assessment of reserve relies mainly on anti-Müllerian hormone (AMH) and the antral follicle count on ultrasound, while FSH plays a supporting role.

For diagnosing menopause and its stages, the clinical picture and hormonal criteria are used, summarised in the STRAW+10 system (Harlow et al., 2012). FSH in perimenopause fluctuates very strongly, so a single value is rarely sufficient.

Against a background of combined hormonal contraception, FSH is suppressed, and the test does not reflect the body's own ovarian function. The doctor must be informed about the intake of such drugs.

For men: FSH and spermatogenesis

In men, FSH is one of the key indicators in an infertility work-up. The AUA/ASRM guideline on male infertility (Schlegel et al., 2021) recommends a hormonal evaluation that includes FSH and testosterone, in particular for men with abnormal spermogram parameters.

Elevated FSH combined with small testicular volume and azoospermia or oligozoospermia usually indicates a primary disorder of spermatogenesis. Low FSH together with low LH and testosterone — central (secondary) hypogonadism, including as a result of taking exogenous androgens.

For men who have used anabolic steroids, FSH and LH are often suppressed for a long time after stopping intake. It is important to report this to the andrologist, because the further examination strategy depends on it.

The spermogram, which is often ordered together with FSH, is performed according to the standards of the WHO laboratory manual (6th edition, 2021), which includes its own preparation requirements, in particular a period of sexual abstinence.

General rules and influencing factors

For FSH itself, strict fasting is not so critical, but it is almost always taken in a panel with estradiol, LH, testosterone or prolactin. Therefore unified conditions make life easier and increase the comparability of results.

• for women — day 2–5 of the cycle, unless the doctor has indicated otherwise; indicate the date of the last menstruation;
• for men — a morning draw, fasting, if testosterone is taken together;
• the day before — without intense training, alcohol, with normal sleep;
• do not take it during an acute illness;
• report hormonal drugs, contraceptives, androgens, ovulation stimulation;
• report biotin intake.

Biotin in high doses may distort FSH immunoassays; the FDA has separately warned about this interference. Check the length of the pause before the test with the laboratory.

Hormonal drugs, gonadotropins, GnRH analogues, androgens and estrogens significantly alter FSH. You should not discontinue therapy on your own — the doctor must know against what background the test was done.

A pronounced energy deficit, excessive training, sharp weight loss and hyperprolactinemia lower gonadotropins. For female athletes with cycle disorders this is especially relevant: FSH and LH taken during a period of intense preparation may reflect precisely an energy deficit.

Limitations of the test and repeat measurements

FSH varies from cycle to cycle, so a single result, especially a borderline one, is rarely grounds for final conclusions. A repeat test under similar conditions and in the same laboratory provides more information.

Results obtained by different test systems may differ. Reference intervals for women are given separately for the cycle phases and postmenopause, and for men — separately. Rely on your laboratory's norms.

A normal FSH does not guarantee fertility, and an elevated one does not always mean the impossibility of pregnancy. Therefore the assessment of reproductive potential combines hormones, ultrasound, spermogram and the clinical picture.

The units of measurement are IU/L or mIU/mL, which are numerically equal. This is one of the few situations where unit conversion is not needed.

Editorial conclusions

FSH is an important marker of the work of the sex glands, but its informativeness depends on the correct day of the draw and the accompanying indicators. For women, baseline FSH is taken on day 2–5 of the cycle together with estradiol; for men — in the morning together with testosterone and LH.

Hormonal drugs, contraceptives, androgens, energy deficit and biotin can alter the result, so the doctor must be informed about them.

Conclusions about fertility or menopause are made by a doctor based on the totality of data, rather than on a single figure.

We also advise reading our articles on preparing for the LH, estradiol and progesterone tests.

References

1. Harlow SD, Gass M, Hall JE, et al. Executive summary of the Stages of Reproductive Aging Workshop + 10: addressing the unfinished agenda of staging reproductive aging. J Clin Endocrinol Metab. 2012;97(4):1159–1168.
2. Practice Committee of the American Society for Reproductive Medicine. Testing and interpreting measures of ovarian reserve: a committee opinion. Fertil Steril. 2020.
3. Schlegel PN, Sigman M, Collura B, et al. Diagnosis and treatment of infertility in men: AUA/ASRM guideline. J Urol. 2021.
4. World Health Organization. WHO laboratory manual for the examination and processing of human semen. 6th ed. Geneva: WHO; 2021.
5. 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.
6. U.S. Food and Drug Administration. The FDA warns that biotin may interfere with lab tests: FDA safety communication. 2017.