T4 suppression test involving 24-hour thyroidal 131I uptake in patients with Graves' disease compared to the T3 suppression test.

A T4 suppression test involving 24-h thyroidal 131I uptake was carried out on patients with Graves' disease during therapy with an antithyroid drug. Thirty-three patients received propylthiouracil (PTU) for at least 1 year. Each patient was given 75 micrograms L-T3 daily for 8 days in conjunction with PTU (50 mg/day) at the time of the experiment and then the 24-h thyroidal 131I uptake (post T3 uptake was measured). Twenty-two patients had normal levels of serum T3 and T4-I before L-T3 administration and were divided into 2 groups, positive T3 suppression (post T3 uptake less than or equal to 35%, group I) and negative T3 suppression (post T3 uptake greater than 35%, group II). Eleven patients showed elevated serum T3 or T4-I concentrations before L-T3 administration (group III). The T4 suppression test was then performed on the same patients. Each patient was given 300 micrograms L-T4 daily for 8 days in conjunction with PTU and the 24-h thyroidal 131I uptake (post T4 uptake) was measured. Some patients receiving PTU (50 mg/day) were switched to MMI (5 mg/day) and the T4 suppression test was done one month later. A significant correlation between the two suppression tests during PTU therapy was observed (r = 0.961, p less than 0.01). None of the patients complained of side effects during the T4 suppression test. The mean post T4 uptake in group I was 18.8 +/- 3.3% during PTU therapy and 5.4 +/- 1.3% during MMI therapy.(ABSTRACT TRUNCATED AT 250 WORDS)