Chemical hyperthyroidism: serum triiodothyronine levels in clinically euthyroid individuals treated with levothyroxine

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Abstract
Many patients treated with levothyroxine sodium who have elevated serum thyroxine (T4) levels but appear clinically euthyroid have been observed. Such patients generally have normal serum triiodothyronine (T3) values. A retrospective review comparing the correlation of T3 and T4 values in levothyroxine-treated patients with that in patients not so treated was carried out from 1977-1979. Mean free T4 index (FTI) value in 104 levothyroxine-treated patients was 4.70 .+-. 0.2 and mean T3 value was 177 .+-. 9 ng/dl. In a group of 50 hyperthyroid patients, mean FTI value was 7.26 .+-. 0.5, with a mean T3 value of 389 .+-. 26 ng/dl. In 71 euthyroid patients, mean FTI value was 2.36 .+-. 0.1, with a T3 value of 137 .+-. 3 ng/dl. Computed ratios of T3 to FTI and T3 to T4 were significantly lower in the group treated with levothyroxine than in the hyperthyroid or euthyroid nontreated groups. Levothyroxine-treated patients with high T4 levels but normal T3 levels were clinically euthyroid. Patients not treated with levothyroxine with similarly elevated T4 levels had elevated T3 levels and were clinically hyperthyroid. Lower relative T3 levels in levothyroxine-treated patients may explain why these patients appear clinically euthyroid despite elevated T4 values. Serum T3 determination is the procedure of choice for evaluation of levothyroxine-treated individuals. An elevated FTI value in such an individual does not, in itself, dictate need to reduce dosage.