Amiodarone Inhibits the Conversion of Thyroxine to Triiodothyronine in Isolated Rat Hepatocytes

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Abstract
Previous studies have shown decreased T3 [triiodothyronine] and increased T4 [thyroxine] and rT3 serum levels in subjects treated with amiodarone. The acute effect of amiodarone on the in vitro conversion of T4 to T3 in suspensions of isolated rat hepatocytes was studied. Iodine and melperone, another class III antiarrhythmic drug, were included in the study as controls. Amiodarone (60 .mu.M) totally blocked the formation of T3 from T4, whereas concentrations of 6 and 0.6 .mu.M reduced T3 formation to 13.7 .+-. 10.6% and 63.9 .+-. 15.9% (.+-. SD), respectively, compared with untreated controls. The drug solvent did not affect the conversion rate. Iodide (120 .mu.M) had an inhibitory effect of .apprx. 10% compared with the controls, whereas melperone did not affect this in vitro conversion. Amiodarone (60 .mu.M) caused a slight but significant reduction of the cellular uptake of [125I]T4 after 10 min of incubation, whereas the 60 min values were unaltered. Amiodarone apparently inhibits the 5''-monodeiodination of T4 to T3 in a dose-related manner. The antiarrhythmic activity of the drug apparently is related to its inhibitory effect on the conversion of T4 to T3.

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