Elevated triiodothyronine and dextrothyroxine levels: a potential cause of iatrogenic hyperthyroidism

A woman in whom the use of dextrothyroxine was associated with clinical hyperthyroidism is described. She had a markedly elevated T3 level while receiving the drug, apparently resulting from conversion of the dextrothyroxine to triiodothyronine. The T3 levels fell to normal after withdrawal. Patients receiving dextrothyroxine should be examined for possible elevation of T4 and T3 thyrotoxicosis.