Plasma testosterone and sex-hormone-binding globulin capacity in physical exercise.

Plasma testosterone (T) levels, sex-hormone-binding globulin (SHBG) capacities and T/SHBG ratios were measured in fifteen athletes during and after short- and long-term running exercises. Plasma SHBG did not change either during or after the running tests. In contrast, exercises of high intensity resulted in a slightly higher T/SHBG ratio during the exercise and a significantly lower T/SHBG ratio a few hours after the exercise. No difference in mean plasma SHBG capacity was found between trained and untrained men. This study indicates that increases or decreases in total plasma testosterone levels in exercise are associated with parallel changes in the free testosterone fraction.