Inhibin-B

Inhibin-B is secreted by the granulosa cells of small antral follicles during the early follicular phase. It provides negative feedback to the pituitary to reduce FSH secretion. As ovarian reserve declines, fewer follicles produce inhibin-B, so FSH rises (the basis of the day-3 FSH test).

Inhibin-B is considered a less reliable standalone marker than AMH or AFC because levels fluctuate more across cycles and laboratory assays vary. It is most useful as part of a multi-marker panel when AMH results are borderline or unavailable. Low inhibin-B in men is associated with impaired spermatogenesis and can help distinguish obstructive from non-obstructive azoospermia.