Thyroid hormone-induced changes in body temperature and metabolism during exercise in dogs

Changes in serum total thyroxine concentration (TT4) and the effects of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) injection on plasma free fatty acid (FFA) level and rectal temperature (Tre) responses were measured in six dogs at rest and during 1 h of submaximal treadmill exercise. At rest there were no increases in FFA level or Tre up to 72 h after thyroid hormone treatment. During exercise, 5 h after a single T4 injection (0.1 mg/kg), there was a) a significant increase in TT4, although the resting level was markedly elevated, and b) a significant increase in FFA concentration and Tre above control values. Seventy-two hours after T4 injection there was a similar increase in TT4 during exercise and both FFA and Tre levels were greater than 5-h values. The elevated Tre was not associated with increased plasma Na+, K+, or osmotic concentrations. Compared with T4 data, T3 injection (0.1 mg/kg) resulted in greater increases in FFA level and Tre during exercise; two animals reached 43.1 degrees C. There were no significant differences in the respiratory exchange ratio (R) or O2 uptake between the control and T3 experiments. It was concluded that thyroid hormones markedly enhance FFA mobilization and elevated Tre during exercise, but not a rest. The hyperthermic response appears to be due to an increase in the level of regulated body temperature rather than to a depression of heat dissipation.