Short-term influence of 3,5,3'-triiodothyronine infusion on resting metabolic rate of the young pig.

The influence of 3,5,3'-triiodothyronine (T3) on resting metabolic rate (RMR) has been examined in the young pig. A dose of 10, 20 or 40 nmol T3/kg body weight was infused intravenously for periods of between one and 4 h and oxygen consumption and activity were measured over the following 20 h. There was no change in RMR during the period of T3 infusion. Results 12-20 h after infusion were influenced neither by the time at which the animal was fed nor by the length of infusion. Immediately after infusion there was a marked increase in plasma T3 concentration, followed by a rapid decline. Levels were much higher than those recorded previously after feeding. Only after 40 nmol T3/kg was there a significant 12% increase in RMR during the period 12-20 h after infusion (P less than 0.02). The significance of these results in relation to the source of the postprandial increase in plasma T3 and the action of thyroid hormones at the cellular level is discussed.