Effect of triiodothyronine on body temperature at rest and during exercise in dogs.

To determine the effect of the thyroid hormones on body temperature at rest and during exercise, five mongrel dogs were trained to run on a treadmill and then treated with exogenous triiodothyronine (T3) (1 mg/kg) to elevate circulating T3 levels or propylthiouracil (PTU) (200 mg/day for 14 days) to lower endogenous T3 levels. When compared with levels in euthyroid controls at rest and during exercise, rectal temperature (Tre) was increased (P less than or equal to 0.01 and P less than or equal to 0.02, rest and exercise, respectively) in T3-treated dogs and decreased (P less than 0.05 and P less than 0.01) in the PTU-treated dogs. Although the rise in Tre from rest to exercise in the PTU trial was slightly less than that of the control runs (1.23 vs 1.44 degrees C, respectively; P = 0.05), it was similar to the rise in Tre observed in the T3 experiments (1.37 degrees C). Because T3 treatment resulted in an elevation in exercise oxygen consumption (VO2), untreated dogs were run at elevated work loads such that their VO2 matched that recorded when pretreated with T3. Exercise Tre in these experiments was lower (P less than 0.02) than after T3 treatment, even though the exercise metabolic rates were equivalent. It was concluded that 1) although T3 alters body temperature, the magnitude of the rise in Tre during exercise is not dependent on T3 concentration and 2) the effects of T3 on body temperature at rest and during exercise are mediated through mechanisms other than alterations in metabolic rate.