Dronedarone (Dron), without iodine, was developed as an alternative to the 
iodine-containing antiarrhythmic drug amiodarone (AM). AM acts, via its major 
metabolite desethylamiodarone, in vitro and in vivo as a thyroid hormone 
receptor alpha(1) (TRalpha(1)) and TRbeta(1) antagonist. Here we investigate 
whether Dron and/or its metabolite debutyldronedarone inhibit T(3) binding to 
TRalpha(1) and TRbeta(1) in vitro and whether dronedarone behaves similarly to 
amiodarone in vivo. In vitro, Dron had a inhibitory effect of 14% on the binding 
of T(3) to TRalpha(1), but not on TRbeta(1). Desethylamiodarone inhibited T(3) 
binding to TRalpha(1) and TRbeta(1) equally. Debutyldronedarone inhibited T(3) 
binding to TRalpha(1) by 77%, but to TRbeta(1) by only 25%. In vivo, AM 
increased plasma TSH and rT(3), and decreased T(3). Dron decreased T(4) and 
T(3), rT(3) did not change, and TSH fell slightly. Plasma total cholesterol was 
increased by AM, but remained unchanged in Dron-treated animals. 
TRbeta(1)-dependent liver low density lipoprotein receptor protein and type 1 
deiodinase activities decreased in AM-treated, but not in Dron-treated, animals. 
TRalpha(1)-mediated lengthening of the QTc interval was present in both AM- and 
Dron-treated animals. The in vitro and in vivo findings suggest that dronedarone 
via its metabolite debutyldronedarone acts as a TRalpha(1)-selective inhibitor.