Reappraisal of the 3,5,3'-triiodothyronine-suppression test in the prediction of long term outcome of antithyroid drug therapy in patients with hyperthyroid Graves' disease.

Thyroidal suppressibility by exogenous T3 in terms of both radioiodine uptake (RAIU) and serum T4 was evaluated in 115 hyperthyroid patients treated with methimazole for 2 yr and followed for an additional 2 yr to study the rate of recurrence. Various other serum parameters including serum thyroglobulin concentrations, thyroid autoantibody, and TSH receptor antibody titers, and thyroidal responses to TRH-induced TSH elevation were also determined. After 2 yr of methimazole therapy, thyroidal RAIU was not suppressible (RAIU less than 12%/4 h was defined as suppressible) in 50 of 115 patients (group I). Of 65 patients with suppressible thyroid RAIU, serum T4 was significantly reduced (less than 60% of pre-T3 level) by T3 administration in only 43 patients (group III) but not in the remainder (group II). Antithyroid drug therapy was discontinued in the group II and III patients, and 7 of the patients had recurrence of hyperthyroidism within 2 yr of follow-up. All of them were from group II. The thyroidal response to TSH was greater in group III patients than in group II patients. During antithyroid drug therapy, decrease of microsomal antibody titer was more likely to occur in group III patients than in those of group II. Serum thyroglobulin concentrations were uniformly normal in treated patients irrespective of T3 suppressibility. TSH receptor antibody was positive in all 13 untreated patients with Graves' disease but was negative in treated patients regardless of their T3 suppressibility. Measurement of both thyroidal RAIU and serum T4 after administration of T3 improves the reliability of T3-suppression testing as a predictor of the remission of Graves' disease.