Inhibin B

The Inhibin B test (also known as the inhibitor of follicle-stimulating hormone secretion) is employed to assess antral follicle function in women and to evaluate Sertoli cell activity and sperm production in men. In infertility workups, it is interpreted alongside FSH to estimate ovarian reserve and to characterize male factor infertility. It also aids diagnosis and surveillance of granulosa cell tumors and mucinous epithelial ovarian tumors that overexpress inhibin B. Additional uses include evaluation of abnormal pubertal timing, assessment of disorders of sex development, and distinction between cryptorchidism and anorchia in boys.

Inhibin B is a transforming growth factor-β family hormone composed of α and β subunits joined by disulfide bonds. It exerts negative feedback on pituitary FSH and has local paracrine effects within the gonads. Secretion decreases with gonadotropin-releasing hormone (GnRH) and increases with androgens, FSH, and insulin-like growth factor-1. In women, antral follicles are the principal source. Concentrations rise in the early follicular phase, peak mid-follicular, remain low in the luteal phase, and display a brief surge approximately two days after the LH peak near ovulation. As follicle number declines with age, follicular-phase inhibin B falls earlier than estradiol or inhibin A; postmenopausal values are very low or undetectable, and marked elevations in this setting suggest granulosa cell or mucinous ovarian carcinoma. Combining inhibin B with CA 125 and imaging improves tumor detection and supports treatment monitoring. For ovarian reserve assessment, results are interpreted with FSH and anti-Müllerian hormone, commonly obtained on cycle day 3. In males, inhibin B is a key regulator of FSH. Adult levels are relatively stable, correlate with testicular volume and sperm output, and are decreased in oligospermia, azoospermia, and impaired spermatogenesis. In boys, detectable concentrations indicate the presence and function of testicular tissue and assist in distinguishing anorchia from cryptorchidism. Because concentrations fluctuate across the menstrual cycle, interpretation in premenopausal women is more complex; results require integration with clinical findings and other diagnostic studies and do not constitute definitive evidence for or against malignancy.