To the Editor.—
In the editorial on thyroid dysfunction, Nusynowitz and Young (242:275, 1979) speculated on the possible usefulness of serum reverse triiodothyronine (T3) levels in differentiating sick patients with isolated hyperthyroxinemia (low or normal serum T3 values) from sick patients with hyperthyroidism. We measured serum reverse T3 concentrations in the group of sick patients described in the same issue of The Journal (242:251, 1979).1 The mean serum reverse T3 concentration was significantly higher in the study group, at 65±20 ng/dL, vs 24±5 ng/dL (P<.05) in normal control subjects. Segregation of the sick patients into the "euthyroid sick" and "hyperthyroid sick" groups, however, revealed no significant difference in the reverse T3 means (61±20 ng/dL and 43±7 ng/dL, respectively). Thus, serum reverse T3 values seem to be of no value in distinguishing sick euthyroid patients with high serum free thyroxine indices from sick hyperthyroid patients.