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 lenght 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 < 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.