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

Summary 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 × 400 mg daily for 3 days in addition to methimazole, 3 × 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 µg/dl and 507 pg/dl, respectively in Group I and 377 ng/dl, 17.6 µg/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.