Abstract Various metabolic parameters were studied in a group of obese individuals and individuals of normal weight who were euthyroid by the usual clinical and laboratory criteria. The parameters studied were the blood glucose replacement rate, excretion of C 14O 2 in expired air following the injection of 1-C 14-glucose and 6-C 14-glucose, the hexosemonophosphate shunt, the biologic half-life of I 131-T 3 and the concentration of C 14-T 3 in bile and adipose tissue. Studies were obtained before and after a period of triiodothyronine administration. Loss of weight occurred without manifestations of thyrotoxicosis. The blood glucose replacement rate mg./Kg./hr. was increased in obese individuals by T 3. The increase was 20 to 50 per cent of the pretreatment level. Following T 3 administration there was an increase in the respiratory excretion of C 14O 2 after injections of 1-C 14-glucose and 6-C 14-glucose. The pentose pathway showed a slight decrease after T 3 treatment. The biologic half-life of I 131-T 3 (1.6 days) was shorter in obese individuals than in those of normal weight (2.9 days). After T 3 therapy the half-life increased toward normal values. The C 14-T 3 content in bile was increased in obese individuals suggesting an increased excretory mechanism. Levels of T 3/Gm. adipose tissue were about one-fourth of the value in obese patients than in those of normal weight, but the total fat content of obese patients was 6–8 times that of the normal weight subjects.