Inhibin B Test: Normal Ranges, Fertility & Ovarian Reserve

What Is Inhibin B?

I often see patients who have been told their ovarian reserve is ‘low’ but don't fully understand the tests behind that statement. Inhibin B is a hormone produced by the granulosa cells in women's ovaries and by the Sertoli cells in men's testes. Its primary job is to regulate the pituitary gland's secretion of follicle-stimulating hormone (FSH) – think of it as a fine-tuning dial that keeps FSH from rising too high.

In women, inhibin B levels fluctuate across the menstrual cycle, peaking in the early follicular phase. In men, levels are relatively stable and reflect Sertoli cell function, which is essential for normal sperm production. Measuring inhibin B gives us a real-time snapshot of gonadal activity.

Why Is Inhibin B Measured?

In my clinical practice, the main indications for ordering inhibin B include:

• Assessment of ovarian reserve – together with anti-Müllerian hormone (AMH) and antral follicle count, it helps predict a woman's remaining egg supply.
• Evaluation of male infertility – a low inhibin B in men can point to Sertoli cell dysfunction or impaired spermatogenesis.
• Monitoring pubertal development – particularly in boys with delayed or precocious puberty.
• Detecting ovarian granulosa cell tumours – this tumour secretes large amounts of inhibin B and can be used as a tumour marker.

Normal Reference Ranges for Inhibin B

Reference ranges vary by laboratory and assay, but the table below gives approximate values that I use in clinic. Always interpret results in context – a single number rarely tells the whole story.

Inhibin B and Female Fertility

Can Inhibin B Predict Menopause?

A common question I hear is, “Will this test tell me when I’ll hit menopause?”. Inhibin B declines years before the final menstrual period, and a low level (especially <20 pg/mL in the early follicular phase) strongly suggests diminished ovarian reserve. However, it is not a crystal ball – menstrual patterns and AMH are more reliable for predicting the exact timing of menopause.

Inhibin B During Pregnancy

In pregnancy, inhibin B is produced by the placenta and foetal ovaries. Levels rise dramatically in the first trimester and then fall. Clinically, second-trimester inhibin B is part of the quadruple screening test for Down syndrome, but isolated inhibin B is rarely used alone for that purpose.

Inhibin B and PCOS

Women with polycystic ovary syndrome (PCOS) often have higher inhibin B levels due to an increased number of small antral follicles. This can paradoxically suppress FSH and contribute to anovulation. In my practice, I use inhibin B alongside AMH to differentiate PCOS from other causes of oligomenorrhoea.

Inhibin B in Men

What Does Low Inhibin B Mean for Male Fertility?

In men, inhibin B is a direct marker of Sertoli cell mass and, by extension, spermatogenesis. A level below 80–100 pg/mL is often associated with reduced sperm counts (<15 million/mL). I tell my patients that a low inhibin B is not a death sentence for fertility – it warrants a full semen analysis and possibly genetic testing for Y-chromosome microdeletions.

Factors That Affect Inhibin B Levels

• Age – declines in both sexes, but more steeply in women after 35.
• Menstrual cycle day – always interpret women's results against cycle timing.
• Oral contraceptive pills – suppress inhibin B production.
• Gonadotropin therapy – can transiently raise inhibin B.
• Ovarian or testicular tumours – may cause extreme elevations.

Limitations of the Inhibin B Test

No test is perfect. Inhibin B has significant cycle-to-cycle variability in women, and assays from different laboratories may not be directly comparable. It should never be used in isolation – I always combine it with AMH, FSH, and ultrasound findings. In men, inhibin B can be normal even with obstructive azoospermia, so it is not a substitute for semen analysis.

Inhibin B vs. AMH: What’s the Difference?

Both are markers of ovarian reserve, but AMH is more stable across the menstrual cycle and is currently the preferred first-line test. Inhibin B adds information about the immediate follicular cohort – it is like comparing a short-term weather forecast (inhibin B) to the long-term climate trend (AMH). In my practice, I order both when a patient’s situation is complex or when results are borderline.

Disclaimer: This article is for educational purposes only and does not replace a consultation with your healthcare provider. Reference ranges may differ between laboratories.