Five normal men were made clinically hyperthyroid by administration of T3, 300 μg daily for three weeks. In 3 men, estradiol metabolic clearance rates and plasma concentration of testosterone-estradiol binding globulin (TeBG) were measured during the control periods, during fluoxymesterone administration, and during hyperthyroidism. Plasma LH and testosterone concentrations were determined throughout all studies in 5 men. Treatment with T3 caused a 7–8 fold increase in TeBG; a 2–3 fold increase in plasma testosterone; no change in CBG, and no change in in 2 patients and a 25% decrease in the third. The marked rise in TeBG during T3 treatment was not accompanied by a corresponding large fall in hence it is most likely that hyperthyroidism increased the rate of metabolism of estradiol. In spite of a 2–3 fold increase in plasma testosterone, plasma LH increased during T3 treatment, indicating that it is the non-TeBG-bound testosterone that determines LH release. The plasma half-life of TeBG was estimated at 5.1 days by following its rate of decrease in plasma after stopping T3. During fluoxymesterone treatment, when TeBG and plasma testosterone were decreased, rose 18%.