Circulating immunoactive inhibin in the luteal phase and early gestation of women undergoing ovulation induction.

Serum inhibin was measured by radioimmunoassay in 19 women undergoing ovulation induction within an in vitro fertilization program. In 16 nonpregnant subjects, inhibin rose during the luteal phase to reach a peak (2.5 U/ml) 6 days after oocyte retrieval; it then fell to undetectable levels by day 14 (less than 0.37 U/ml). Serum inhibin was significantly correlated (P less than 0.001) with both serum progesterone and estradiol concentrations and inversely correlated with serum follicle-stimulating hormone (FSH) (P less than 0.001). In the pregnant group (n = 3), inhibin levels reached an initial peak (3.7 U/ml) on day 6, then declined to a nadir (1.5 U/ml) on day 8 after oocyte retrieval and subsequently rose to 3.4 U/ml on day 14. These data suggest (1) an inverse relationship between inhibin and FSH, which is in support of the inhibin hypothesis; (2) that the human corpus luteum secretes inhibin; and (3) a trophoblastic/placental origin of inhibin during gestation.