Ability to perform prolonged physical exercise in dogs after chronic treatment with thyroxine.

Treatment of dogs with L-thyroxine (T4) for 7 or 21 days had no influence on their body temperature, blood LA and NA concentrations and muscle glycogen content at rest. Only blood FFA and glucose concentrations were significantly higher than those before T4 administration. During exercise performed by T4-treated dogs considerably higher increases in rectal temperature as well as in blood FFA and LA concentrations were found in comparison with control runs. Plasma NA level at the end of exercise was significantly lower in T4-injected dogs than in controls, and blood glucose and muscle glycogen content were similarly decreased in spite of much shorter time of exercise until exhaustion in T4-treated dogs. As a result of the described metabolic and temperature alterations ability of T4-treated dogs to perform prolonged physical exercise was markedly reduced.