‘Chemical Hyperthyroidism’: Serum Triiodothyronine Levels in Clinically Euthyroid Individuals Treated with Levothyroxine

We have observed many patients treated with levothyroxine sodium who have elevated serum thyroxine (T₄) levels but appear clinically euthyroid. Such patients generally have normal serum triiodothyronine (T₃) values. A retrospective review at The Johns Hopkins Hospital, Baltimore, comparing the correlation of T₃ and T₄ values in levothyroxine-treated patients with that in patients not so treated was carried out from 1977 to 1979. Mean free thyroxine 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 either 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. It is concluded that 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. Furthermore, an elevated FTI value in such an individual does not, in itself, dictate need to reduce dosage.