Effect of amiodarone on serum T4 and T3 levels in hyperthyroid patients treated with methimazole

We investigated the course of thyroid hormones levels in the serum of hyperthyroid patients acutely treated with amiodarone. Ten patients were treated either with amiodarone, 3 X 400 mg daily for 3 days in addition to methimazole, 3 X 20 mg daily for 10 days (Group I; n = 5) or with a placebo plus methimazole at the same doses (Group II; n = 5). Basal T3, T4 and rT3 serum concentrations were: 297 ng/dl, 16.6 micrograms/dl and 507 pg/dl, respectively in Group I and 377 ng/dl, 17.6 micrograms/dl and 362 pg/dl in Group II (NS). Compared with basal values, the drop in serum T3 concentration became significant on Day 1 in Group I, but not until day 5 in Group II. The decrease in serum T3 concentration was significantly higher in Group I than in Group II from Day 1 to Day 7. In Group I, T4 concentration was significantly lower on Days 2, 4 and 6; the percentage drop in T4 calculated from the areas under the curves was higher and the T3/T4 ratio lower on Days 3-5, 7 and 9; rT3 was higher on Days 4 and 5 and its rise was significant on Days 1, 3 and 4. During the follow-up period a transient rise in T4 and T3 concentrations was observed in two patients in Group I when the methimazole dosage was tapered or stopped because of agranulocytosis. In conclusion, in our hyperthyroid patients, amiodarone in conjunction with methimazole induced a greater fall in T3 and T4 than did methimazole alone.