The effects of various doses of conjugated estrogens (Premarin®) on modern thyroid function tests were studied in 14 euthyroid menopausal women before and after 6 weeks of therapy with each of these daily doses: 0.625 mg, 1.25 mg, 2.5 mg, and, again, 0.625 mg. Serum thyroxine, triiodothyronine, thyroxine-binding globulin, and thyrotrophin were measured by radioimmunoassay, free thyroxine by dialysis, and triiodothyronine binding index by a conventional method. Increasing doses of conjugated estrogens led to stepwise elevations of thyroxine, triiodothyronine, thyroxine-binding globulin, and the binding index, with a threshold dose below 0.625 mg/day. The peak effect seemed to be between 1.25 and 2.5 mg/day. Free thyroxine and thyrotrophin concentrations did not change at any dose.