Inhibin B

The Inhibin B test (also referred to as the inhibitor of follicle-stimulating hormone secretion) is used to assess antral follicle function and ovarian reserve in women and to evaluate Sertoli cell function and spermatogenic activity in men. In infertility workups, results are interpreted alongside follicle-stimulating hormone to inform prognosis and guide assisted reproductive technology decision-making. In oncology settings, serum inhibin B aids in the diagnosis and longitudinal monitoring of granulosa cell tumors and mucinous epithelial ovarian tumors known to overexpress this marker. The assay also supports evaluation of pubertal timing disorders and disorders of gonadal development, and helps distinguish cryptorchidism from anorchia.

Inhibin B is a TGF-β superfamily glycoprotein synthesized by Sertoli cells in the testis and granulosa cells in the ovary. It participates in negative feedback on pituitary follicle-stimulating hormone and also acts locally through paracrine mechanisms; its secretion is stimulated by follicle-stimulating hormone and androgens. In females, inhibin B is produced by antral follicles. Concentrations rise in the early follicular phase, reach a mid-cycle peak with a transient surge around ovulation, and are low in the luteal phase. Levels decline with age and become very low or undetectable after menopause. Marked elevations in postmenopausal women suggest granulosa cell or mucinous epithelial ovarian carcinoma, and values can exceed baseline by more than 60-fold. When evaluating such neoplasms, combining inhibin B with CA 125 and imaging improves assessment and follow-up. On cycle day 3, inhibin B reflects ovarian reserve and is interpreted with follicle-stimulating hormone and anti-Müllerian hormone, including in assisted reproduction settings. In males, circulating inhibin B mirrors Sertoli cell function and spermatogenesis, correlating with testicular volume and sperm concentration. Concentrations are reduced in oligospermia, azoospermia, and other forms of impaired spermatogenesis. The marker helps distinguish anorchia from cryptorchidism and contributes to the evaluation of disorders of sex development. Analytical considerations are critical: results produced by different assay platforms are not interchangeable. Values vary with the menstrual cycle, and inhibin B should not be used as the sole criterion to confirm or exclude malignancy.