Immunoreactive Inhibin Concentrations in Adult Men: Presence of a Circadian Rhythm

The circadian and chronological changes in inhibin secretion were studied in normal adult men. To determine the diurnal release of inhibin, blood samples were collected every hour for 24 h from five healthy young adult men, and serum inhibin was measured by RIA. During the evening and the night, serum inhibin concentrations were relatively low; the lowest value was observed at 2200 h. At 0700 h, inhibin started to increase, reached a peak at 0900 h, and then gradually decreased. These results suggest that inhibin secretion is circadian. Testosterone and cortisol also showed circadian rhythms. In some of the five volunteers, the serum concentrations of inhibin, testosterone, and cortisol were superimposable, but no significant relationship was observed between the serum inhibin and FSH concentrations. With regard to the age-related changes in basal inhibin levels, the highest values were observed in the twenties, and lower values were found with aging. This relationship suggests that increased FSH in elderly men might be due to the reduced amount of peripheral inhibin.