Inhibin is absent from azoospermic semen of infertile men

There is ample evidence that the secretion of follicle stimulating hormone (FSH) by the pituitary gland is regulated by a nonsteroidal hormone of gonadal origin, termed inhibin. In several species, testicular extracts, gonadal lymph and seminal plasma have been shown to contain proteins which inhibit FSH release; lack of suitable assays for inhibin has, however, prevented clear definition of its physiological significance. Men whose tests show the histological changes of germinal cell failure usually have raised FSH levels in the blood, but may have normal levels of luteinizing hormone (LH) and testosterone. Such patients would thus be predicted to show reduced inhibin production. To test this, we have measured the FSH inhibitory activity of seminal plasma from azoospermic subjects with raised plasma FSH levels. We report here our confirmation of diminished inhibin levels in seminal plasma of these patients, thus providing convincing support for its physiological role as a modulator of FSH production in man.