Acute effects of amiodarone administration on thyroid function in patients with cardiac arrhythmia

Because little has been published on early effects of treatment with amiodarone on thyroid function, we studied serum total and free thyroid hormone, reverse T-3, and TSH levels in patients with cardiac arrhythmias during the first 10 days of treatment with a loading dose of amiodarone by iv infusion. Twenty-four patients were enrolled in the study. A standardized loading regimen for the iv infusion of amiodarone was used. The protocol provided the iv infusion of 20 mg/kg per day on day 1, the iv infusion of 10 mg/kg per day on day 2, then 600 mg/day per os for 7-10 days, and finally, in patients chronically treated with the drug, the dose was gradually reduced to 400-200 mg/day per os. Total and free concentrations of T-4 tended to progressively and significantly increase (P < 0.0001 repeated measures ANOVA) starting from the fourth day of therapy, whereas total T-3 decreased from the second day progressively (P < 0.0001) throughout the study; free T-3 did not significantly change. TSH levels early and significantly (P < 0.001, by ANOVA) increased throughout the study, starting from the first day of therapy and reaching at 10 days a value 2.7 times higher than the basal value. Reverse T-3 levels progressively and significantly (after 2 days of treatment) increased and paralleled the TSH values, reaching at the 10th day a value about 2 times higher than basal value. In conclusion, our data suggest that after iv treatment with amiodarone: 1) TSH is the first hormone to change significantly followed by reverse T-3, T-4, and T-3; 2) the progressive fall of T-3 levels reflects an inhibition of the peripheral conversion of T-4 to T-3; 3) the observed later increase of total and free T-4 levels may be explained by a contribution of direct thyroidal stimulation by TSH and/or by a reduction in T-4 clearance.