Inhibin B

Inhibin B is a hormone produced by the granulosa cells of the ovary in women and by the Sertoli cells of the testis in men. Its job is to give feedback to the pituitary gland, damping down the release of FSH — so inhibin B and FSH move in opposite directions and are best read together. In women, inhibin B is made by the pool of small growing follicles, so its level reflects ovarian reserve — the number of eggs remaining. As a woman ages and that pool shrinks, inhibin B falls and FSH rises, a pattern seen in diminished ovarian reserve and around the menopause. It is one of several markers (with AMH and FSH) used in fertility assessment, though AMH has largely become the preferred, more stable measure of reserve. In men, inhibin B reflects the health of the sperm-producing tissue: a good level suggests active spermatogenesis, while a low level with a high FSH points to testicular failure. It is also used in monitoring certain ovarian tumours (granulosa cell tumours), which can secrete it. Because it varies so much with sex, menstrual cycle phase and age, inhibin B is always interpreted in context rather than against a single number.

• A granulosa cell tumour of the ovary — these can secrete inhibin B, which is used as a tumour marker to monitor them.
• A good pool of growing follicles — in the fertility context, a higher level reflects better ovarian reserve.
• Active testicular function in men — a healthy level supports normal sperm production.

• Diminished ovarian reserve — fewer remaining eggs; inhibin B falls as FSH rises.
• Approaching or established menopause — the ovarian pool is depleting.
• Testicular failure in men — low inhibin B with a high FSH suggests impaired sperm production.
• Suppression of the pituitary–gonadal axis — from certain hormonal states or treatments.