Hyperthyroxinemia with bradycardia and normal thyrotropin secretion after chronic amiodarone administration.

Pituitary-thyroid function was tested in 15 euthyroid patients before, during, and after long term oral treatment with amiodarone (2-n-butyl-3,4'-diethylaminoethoxy-3',5'-diiodobenzoylbenzofurane; 600-1200 mg daily), an iodine-containing potent antiarrhythmic drug. The drug caused increases in serum total T4, free T4, and rT3, with a concomitant decrease in T3. Baseline serum TSH was significantly higher after 1 week of drug treatment and returned to normal levels after 12 weeks of treatment. All patients receiving amiodarone had a slowing of their heart rate (P less than 0.01), and heart rate gradually increased 6 weeks after drug withdrawal, concurrent with the slow fall in T4 and rT3 levels. Amiodarone did not cross-react in the iodothyronine RIAs. The results suggest that amiodarone inhibits the peripheral conversion of T4 to T3 and may block the metabolic action of thyroid hormone in man.