The relative discriminatory value of the estimation of the serum reverse-T3 levels for the laboratory diagnosis of hyperthyroidism was investigated in 47 patients with clinical signs or symptoms of hyperthyroidism. The results were compared with those from the determination of the total serum levels of T3 and T4 prior and after correction for the binding proteins. Twenty-three of the patients had normal thyroid function and 24 had hyperthyroidism. The estimation of the total serum T3 level was superior to both the determination of the total serum T4 and reverse-T3 levels even subsequent to correction for the binding proteins.