The effects of spontaneous and experimentally induced congestive heart failure 
on serum thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3'5'-triiodothyronine 
(reverse T3), free T4, free T3 concentrations, and the serum T4 and T3 
concentrations in response to administration of thyrotropin were studied. Serum 
thyroid hormone concentrations were not different between eight dogs with 
spontaneous congestive heart failure and normal age matched control dogs. Seven 
dogs with experimental heart failure were tested before and after induction of 
congestive heart failure by rapid ventricular pacing. Mean serum T4 and free T3 
concentrations were decreased and mean serum reverse T3 concentration was 
increased following induction of heart failure. The serum T4 and T3 responses to 
thyrotropin were not altered. Thyroid gland morphology appeared normal in dogs 
with experimental heart failure. Experimental congestive heart failure, similar 
to some other nonthyroidal illnesses, alters thyroid hormone secretion and 
metabolism in dogs.