The interrelationships between serum levels of amiodarone, desethylamiodarone, and reverse T 3, and changes in the corrected QT interval (ΔQT c) were examined in 22 patients during long-term treatment with amiodarone. At 1, 3, and 6 months of follow-up, the correlation coefficient between serum levels of amiodarone or desethylamiodarone and reverse T 3 ranged from 0.01 to −0.2 ( p > 0.4). At the same time intervals, the correlation coefficient between both amiodarone and desethylamiodarone levels and ΔQT c ranged from 0.1 to −0.1 ( p > 0.6), and the correlation coefficient between reverse T 3 and ΔQT c also ranged between 0.1 to −0.1 ( p> 0.5). Substituting percent ΔQT c for ΔQT c also did not reveal a significant correlation. These data demonstrate that serum levels of reverse T 3 cannot be used as a substitute for serum levels of amiodarone in monitoring patients being treated with amiodarone. The absence of a correlation between serum reverse T 3 levels and ΔQT c suggests that the delay in repolarization which occurs during amiodarone therapy is not secondary to an amiodarone-induced abnormality in thyroid hormone metabolism.