In 42 postmenopausal women with breast cancer, aged 48-85 years (mean age 62.4 years) serum thyroid hormone concentrations were measured before and after 6 months of tamoxifen therapy (20 mg daily). In particular triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4), thyroxine-binding globulin (TBG) and thyroid-stimulating hormone (TSH) concentrations before and 30 minutes after thyrotrophin-releasing hormone (TRH) administration (200 microg i.v.) were measured before and 6 months after tamoxifen therapy. T3 and T4 concentrations increased significantly (p<0.001 and p<0.05, respectively) whereas FT3 and FT4 remained unchanged (p>0.05), TBG increased significantly (p<0.001) and basal TSH concentrations as well as TSH response to TRH injection increased significantly (p<0.05) after tamoxifen therapy. It is concluded that tamoxifen administration changes thyroid hormone concentrations. However free thyroid hormone levels remain unchanged and the patients remain euthyroid after long-term tamoxifen therapy.