inhibin

Inhibin is a hormone made by the gonads whose job, as its name suggests, is to put a brake on a specific pituitary signal. Of the two gonadotropins, FSH and LH, inhibin selectively dials down FSH, giving the body a way to tune follicle stimulation independently of the rest of the reproductive axis.

In women, inhibin is produced mainly by granulosa cells of growing ovarian follicles and, in its inhibin A form, by the corpus luteum. It is a protein hormone built from two linked subunits. As follicles grow during the follicular phase and pour out inhibin and estradiol, FSH is suppressed — a selective negative feedback that helps ensure usually just one dominant follicle matures while the rest, deprived of FSH support, fall away.

Because its level tracks follicle activity, inhibin is clinically useful as a marker: inhibin B reflects the size of the growing follicle pool and ovarian reserve, while inhibin A is one of the markers in some prenatal screening tests and is elevated in certain ovarian tumors. A caveat is that for everyday assessment of ovarian reserve, anti-Mullerian hormone has largely become the preferred and more stable marker.