Inhibin and steroid responses to testicular stimulation in normal men

Static measurements of immunoreactive inhibin have proved to be of little relevance in the diagnosis of testicular disorders. To explore whether a dynamic evaluation of inhibin secretion might yield a more useful parameter of testicular function we compared the responses of inhibin with steroids to i.v. injections of pure follicle-stimulating hormone (FSH; 300 IU) or human chorionic gonadotrophin (HCG; 1500 IU) and oral administration of the anti-oestrogen Tamoxifen (20 mg/day for 7 days) in four normal fertile men. Blood was aspirated between 1 and 72 h after the injections and daily during Tamoxifen intake. Four controls were injected with physiological saline solution. An additional four men were injected with pure FSH, and blood was taken after 24, 48 and 72 h. Injection of FSH was accompanied by nycthemeral variations of testosterone comparable with those observed in the controls. The concentration of inhibin showed similar nycthemeral variations but a significant increase was observed in all eight cases at 12 noon on days 2 and 3 after FSH injection. HCG injection resulted in the expected biphasic response of testosterone. Inhibin displayed a pronounced increase 18 h after injection but the delayed response after 48 and 72 h was not observed. Tamoxifen intake increased testosterone but not inhibin, and caused a moderate and temporary increase of luteinizing hormone and FSH. It was concluded that primary stimulation both of Leydig cells by HCG and Sertoli cells by FSH increase circulating inhibin.