Studies on the T3 suppression test with reference to the thyroidal 123I uptake in Graves' disease: comparison of 24-hour and 3-hour uptake.

Eighty-three patients with Graves' disease had been treated with methylmercaptoimidazole (MMI). They were prescribed a maintenance dose of antithyroid drug (MMI, 5 mg/day) at the time of a T3 suppression test. The 3-hour and 24-hour thyroidal 123I uptake after T3 administration (75 micrograms/day, 2 weeks) were measured (post T3 uptake). In 38 patients whose post T3 uptake was below 35% in post T3 24-hour uptake, treatment was stopped. The T3 suppression test was then repeated 1 and 3 months later. During a one-year follow up, 26 remained well, while 12 relapsed within 6 to 12 months. We have observed a good correlation between 3-hour uptake and 24-hour uptake of 123I after T3 administration (r = 0.847, p less than 0.001). In 38 patients who showed positive T3 suppression, most patients with MMI withdrawal produced a marked overshoot of post T3 3-hour and 24-hour uptake at one month. Retrospective analysis indicated that there was no significant difference in circulating thyroid hormone levels between remission and relapse groups. The present study provides evidence that 3-hour uptake values are able to be substituted for 24-hour uptake values during a T3 suppression test. In addition, overshoot of thyroidal uptake after antithyroid drug withdrawal was observed in 3-hour values, similar to 24-hour values.