Recurrent Hyperthyroidism Presenting as Triiodothyronine Toxicosis

Ten cases of triiodothyronine (T3) toxicosis have been identified in patients with prior histories of hyperthyroidism. In each case the recurrence came after a euthyroid interval of from 4 months to 30 years. Recurrence in all patients was characterized by normal serum concentrations of total and free thyroxine but significantly increased levels of total and free T3. Total T3 concentrations ranged from 210 to 2000 ng/100 ml (normal, 96 to 172 ng/100 ml), and free T3 levels varied from 0.5 to 4.0 ng/100 ml (normal, 0.1 to 0.4 ng/100 ml). The possibility of T3 toxicosis must be considered in patients whose clinical picture suggests a recurrence of their hyperthyroidism but in whom serum thyroxine levels are normal. Newly discovered methods for measurement of T3 should permit more frequent identification of this syndrome.