To investigate whether endurance running in men produced basal hormonal changes similar to those reported in women, we obtained blood samples from 31 men running at least 64 km each week and 18 sedentary controls for measurement of levels of total testosterone, non-sex hormone-binding globulin-bound and free testosterone, luteinizing hormone, follicle-stimulating hormone, prolactin, and cortisol. The mean levels of total and nonspecifically bound testosterone as well as prolactin were significantly lower than in controls, although levels remained within the physiological range. Other hormone levels were similar in both groups. The lowered testosterone and prolactin levels parallel the changes reported in women runners.
Wheeler GD, Wall SR, Belcastro AN, Cumming DC. Reduced Serum Testosterone and Prolactin Levels in Male Distance Runners. JAMA. 1984;252(4):514-516. doi:10.1001/jama.1984.03350040044020