Decrease in serum testosterone concentration during treatment with tetracycline.

Serum concentrations of testosterone and the binding capacity of sex hormone binding globulin (SHBG) were measured on days 2 days immediately preceding tetracycline treatment, on 3 days of treatment and on 2 days immediately after cessation of treatment. On the treatment days serum mean testosterone concentrations were significantly lower than on the control days (17 +/- 0.9 vs 21 +/- 0.8 nmol/l, P less than 0.01). There were no differences in the SHBG. The 'free testosterone index' behaved like the total testosterone.