Serum bioactive follicle-stimulating hormone-like activity in human pregnancy is a methodological artifact.

Serum from pregnant women has been shown to contain both FSH-like and FSH antagonistic activities when measured by an in vitro bioassay based upon FSH-dependent aromatase activity of immature rat Sertoli cells. In the present study we further tested the hypothesis that the FSH-like bioactivity of pregnancy serum was due to an authentic aromatase stimulator. A potent inhibitor of aromatase which completely blocked the FSH action on Sertoli cells had no effect on the bioactivity of pregnancy serum. Experiments using conversion of tritiated testosterone to estradiol showed that the factor did not stimulate aromatase activity in rat Sertoli cells. After incubation with pregnancy serum, equally high amounts of estradiol were measurable in the medium in both the absence and presence of Sertoli cells. The activity was almost completely lost after charcoal treatment or ether extraction of the serum and was shown to probably be due to the release of endogenous estrogens by the carrier proteins in the incubation medium of the Sertoli cell assay. These data suggest that the FSH-like bioactivity in serum from pregnant women is an artifact due to nonspecific interference(s) in the bioassay.