Administration of levothyroxine to euthyroid dogs does not affect echocardiographic and electrocardiographic measurements.

The effects of a replacement dose of levothyroxine on electrocardiographic and echocardiographic parameters in euthyroid dogs were examined. Two-dimensional guided M-mode echocardiograms and electrocardiograms were obtained before and after administration of levothyroxine (0.5 mg m-2 twice a day) to 10 euthyroid dogs for a period of eight weeks; four untreated dogs acted as controls. The resting serum total thyroxine concentration was significantly increased in treated dogs during weeks 4 to 8. There were no significant differences in any of the echocardiographic or electrocardiographic measurements resulting from treatment.