The correlation of serum amiodarone levels with abnormalities in the metabolism of thyroxine

Amiodarone, a widely used iodine-containing antiarrhythmic drug, has been shown to divert the peripheral metabolism of T4 towards rT3 than T3. In this prospective study we correlated the concentration of the peripheral thyroid hormones in serum with that of amiodarone. Fifteen euthyroid volunteers were studied, 5 men and 10 women, with a mean age of 64.2 yr, who suffered from various cardiac arrhythmias (atrial fibrillation or multiple ventricular extrasystoles). Serum amiodarone, T4, T3, rT3 and TSH were determined before and 3, 7, and 14 days after the administration of 400-600 mg amiodarone/day. There was a small increase in serum T4 and a suggestive decrease in serum T3. The increase in serum rT3 was significant, with p less than 0.05 for 3 days, p less than 0.01 for 7 days and p less than 0.005 for 14 days. There was a significant correlation between the level of serum amiodarone and the rise in serum rT3 (r = 0.385, p less than 0.05). The rise in rT3 is the main thyroid abnormality after amiodarone administration and may be used as a rough index of the serum concentration of this drug.