Divergent effects of two low-dose oral contraceptives on sex hormone-binding globulin and free testosterone.
The effect of a triphasic combination of ethinyl estradiol and levonorgestrel on the serum concentrations of total testosterone, free testosterone, and sex hormone-binding globulin as measured directly by radioimmunoassay and on the binding capacity of sex hormone-binding globulin was compared with that of a preparation containing ethinyl estradiol and desogestrel. Blood samples were taken on days 6, 11, 21, and 28 of a control cycle, the third cycle of treatment with either ethinyl estradiol-levonorgestrel or ethinyl estradiol-desogestrel (11 volunteers each), the third cycle of a 3-month washout period, and the third treatment cycle after crossover change of the preparations. There was a significant reduction in total testosterone by 16% during treatment with both preparations. Ethinyl estradiol-desogestrel increased the concentration (+175%) and binding capacity (+330%) of sex hormone-binding globulin to a much greater extent than with ethinyl estradiol-levonorgestrel (+92% and +160%). Contrary to this, a significant suppression of non-protein-bound testosterone by 35% was found during treatment with both oral contraceptives. The results demonstrate that an excessive elevation of the levels of sex hormone-binding globulin above the normal range does not cause a corresponding suppression of free testosterone. It is assumed that the decrease in the apparent binding affinity of high sex hormone-binding globulin concentrations to testosterone may be due to protein-protein interactions.