Normalization of peripheral thyroid hormone metabolism induced by successful chronic amiodarone treatment in patients with ventricular arrhythmias

Amiodarone, an antiarrhythmic agent, is also known to have important effects on the peripheral metabolism of thyroid hormones; the relationship between these two effects of the drug, however, is not well established. We tested the hypothesis that the antiarrhythmic effect of amiodarone might be mediated by its effect on the metabolism of thyroid hormones. Peripheral thyroid hormone metabolism was investigated using a double-tracer ([125I]-T4 and [131I]-T3) procedure in 10 normal volunteers and 10 euthyroid patients with complex ventricular arrhythmias before and during 6 months' amiodarone treatment. The underlying cardiac disease was coronary artery disease in four cases, dilated cardiomyopathy in three and idiopathic arrhythmias in three. In all but one patient with complex ventricular arrhythmias amiodarone treatment resulted in a reduction of > or = 80% of premature ventricular contractions and complete suppression of episodes of ventricular pairs or ventricular tachycardia. In all cases successful treatment with amiodarone was accompanied by normalization of all kinetic parameters: T4 to T3 conversion ratio and T3/T4 molar ratio of production decreased to mean values of 24.7 +/- 17.5% and 0.35 +/- 0.22% respectively, whereas T4 production rate increased (mean value 75.9 +/- 30.0 nmol day-1 m-2). Our kinetic data indicate that long-term therapy with amiodarone, when effective in suppressing cardiac arrhythmias, also reduces peripheral T4 to T3 conversion, hence restoring the normal peripheral thyroid hormone metabolic pattern. In conclusion, our study confirms that the antiarrhythmic action of amiodarone may be (at least partially) mediated by its action on thyroid hormone metabolism, and may justify the hypothesis that an altered peripheral metabolism of thyroid hormones may play a role in the pathogenesis of complex ventricular arrhythmias.