Triiodothyronine (T3)-Induced Thermogenesis: Altered T3-Efficiency in Tissues from Fed, Starved, and Refed Hypothyroid Rats*

A microcalorimetric study of perifused tissue samples obtained from hypothyroid thyroidectomized rats killed 15 h after a single injection of T3 was conducted in order to observe at the tissue level, and in a constant environment, the T3-dependent thermogenesis induced in fed, starved, and refed states. When 9 micrograms T3/100 g BW had been administered to fed animals, tissue heat production rate (E) increased in liver, soleus muscle, and myocardium. In kidney cortex an increase in E was observed only after 90 micrograms T3. A dose-response study was performed with liver samples, in which both oxygen consumption and heat production rates were measured. Liver and soleus muscle were then further investigated. T3-dependent thermogenesis could not be demonstrated in tissues obtained from rats which had been starved for 3 days before receiving 9 micrograms T3/100 g BW. After a small carbohydrate intake corresponding to about 60 kJ/rat (refed state; 1 kJ congruent to 0.4 kcal) given over a 3-h period preceding the injection of T3, T3-dependent thermogenesis was again present. It is concluded that differences in T3 dependent thermogenesis similar to those previously observed in animals under different nutritional states continue to exist in vitro, when tissues are no longer exposed to differing levels of humoral factors.