Inhibin B

What it is

Inhibin B is a peptide hormone produced by the granulosa cells of developing ovarian follicles, primarily early antral follicles. During the follicular phase, it exerts negative feedback on the pituitary by inhibiting FSH secretion.

It is the functional counterpart of AMH in hormonal regulation: both are produced by the same cells (granulosa), both reflect the same follicle pool, and both suppress FSH — though through slightly different mechanisms and at slightly different follicular stages.

Why it matters in PCOS

In PCOS, inhibin B is often significantly elevated, for the same reason as AMH: the large number of small antral follicles characteristic of PCOS collectively contributes to high levels. This excess inhibin B contributes to the relative suppression of FSH, which in turn prevents the selection of a single dominant follicle and ovulation.

Inhibin B is used as an ovarian reserve marker, primarily in the context of assisted reproduction. It is measured in the early follicular phase (days 2–4) and reflects the number of recruitable follicles. However, in routine clinical practice, AMH is generally preferred: it is more stable (can be measured at any cycle point), better standardised across laboratories, and better validated as a predictor of ovarian stimulation response.

A low inhibin B (below 40–45 pg/mL in the follicular phase) may signal diminished ovarian reserve. An elevated inhibin B in the context of PCOS does not mean enhanced fertility — only a large number of follicles.

Normal values

• Early follicular phase (days 2–4): 20–150 pg/mL (varies by laboratory)
• Diminished reserve (guidance): < 40–45 pg/mL
• PCOS (typically): often > 100 pg/mL, sometimes > 200 pg/mL
• Luteal phase / menopause: very low (< 10 pg/mL)

When is it measured?

Inhibin B is measured in the early follicular phase (days 2–4), preferably fasting. It is not routinely included in standard PCOS panels (AMH is preferred), but may be requested in a detailed fertility evaluation or in certain assisted reproduction protocols.

Key takeaways

• Produced by granulosa cells — reflects the antral follicle count
• Elevated in PCOS (like AMH) due to large follicle pool
• Suppresses FSH — mechanistically contributes to anovulation in PCOS
• Less used than AMH in routine practice (less stable, less standardised)
• Measured on days 2–4; diminished reserve if < 40 pg/mL