The Effect of Levothyroxine Treatment on Resting Energy Expenditure of Hypothyroid Dogs

Thirty adult, client-owned dogs were diagnosed with hypothyroidism based on history, physical examination findings, hematologic and biochemical abnormalities, thyrotropin (TSH) response testing, endogenous canine thyrotropin (cTSH) concentration, or both, and total serum thryoxine concentration. All dogs received levothyroxine (L-thyroxine) at an initial dose of 22 μg/kg PO sid in either a tablet (13 dogs) or chewable form (17 dogs). Energy expenditure of each dog during apparent rest was estimated with an open-flow indirect calorimetry system by determining the rates of carbon dioxide production and oxygen consumption. Energy expenditure of apparent rest (EE) was lower in untreated hypothyroid dogs compared with reference values for EE. After treatment with L-thyroxine, EE of the hypothyroid dogs was significantly ( P < .05) higher than pretreatment values.