Effect of danazol on thyroid function in postmenopausal women

During treatment with danazol the serum concentration of thyroxin-binding globulin (TBG) decreases. This effect is probably a direct effect on TBG production at the cellular level. In order to exclude an indirect effect on TBG production via the well known suppressing effect of danazol on serum estrogen concentrations, the following study was performed. Twelve healthy female volunteers, who were at least 3 years past the menopause and with serum-estradiol levels below 100 pmol/l, were treated with danazol in dosages of 400, 600 or 800 mg daily. The concentrations of TBG, TSH, total thyroxin (T4), total triiodothyronine (T3) and free dialysable fractions of T4 and T3 were determined, before, and after 2 and 4 weeks of medications. The serum concentrations of TBG, total T4 and T3 decreased. The TSH concentration and the free dialysable fraction of T4 were essentially unaltered after 2 weeks but the TSH had decreased slightly and free T4 increased slightly after 4 weeks. The free dialysable fraction of T3 decreased transiently at 2 weeks. All the observations proved to be independent of the three dosages of danazol applied in this study. In conclusion, danazol treatment influences available tests of thyroid function by reducing the concentration of TBG. This is most probably a direct effect of the drug, and it is clearly independent of the effect on estrogen production. Clinically there is no evidence of a decreased effect of thyroid hormones on the target organs.