STUDIES ON T3 SUPPRESSION TEST IN THYROTOXIC PATIENTS RECEIVING ANTITHYROID DRUGS

Abstract

It is known that 24-hr uptake, not 20-min uptake, is affected by thionamide drugs during thyroid suppression test (ST). On the other hand, previous studies have shown that 24-hr uptake could be used for the ST in patients with thyrotoxicosis receiving thionamide drugs. However, little information is available on the comparison of the effect of antithyroid drugs in the ST by 24-hr uptake.Forty-seven patients were treated with propylthiouracil (PTU, 50-100mg>/day) or methylmercaptoimidazole (MMI, 5-10mg/day) in a maintenance dose for 1-4 years since they became euthyroid (serum TSH : within normal limits). All patients had PTU or MMI even on the day of the measurement of 131I-thyroid uptake (24-hr, %, Up). The first ST was performed : 75μg T3 daily for 8 days given and Up determined. The second ST was assessed on the same patients 4 weeks after either changing PTU into MMI or MMI into PTU. Most of the patients showed a marked depression of post T3 Up during MMI therapy compared to values of PTU therapy. There was no significant difference between the two antithyroid drug treatment in the serum T4, T3 and TSH levels before and after T3 administration. Twenty-five patients showed Up below 35% after T3-ST, regardless of PTU or MMI treatment. Of these 20 remained euthyroid and 5 relapsed within 2 years after withdrawal of antithyroid drugs. The mean post T3 Up in remission was : MMI 4.6±0.7% ; PTU 18.4±3.2%. In contrast, the mean post T3 Up in relapse was : MMI 11.1±3.1% ; PTU 32.1±10.0%.Conclusively, the present study lend to the proposal that criteria of ST by 24-hr Up should be considerd by antithyroid drugs used : MMI less than 5%, PTU less than 20% in the post T3 Up. MMI is more than 10 times effective in depressing the post T3 Up compared to PTU.