Effect of hypothyroidism on tolerance of rats to heat

¹ Department of Physiology, University of Florida, College of Medicine, Gainesville, Florida

Hypothyroid (propylthiouracil-treated) rats show slower rates of rise of both colonic and skin temperatures than do control rats when both groups are restrained and exposed to air at 40 C. The increased tolerance to heat does not appear to be associated with increased heat conductance through skin but rather with lower heat production (oxygen consumption). Metabolic rate of hypothyroid rats decreases slightly during heat exposure while that of controls increases. When both groups are compared at the same colonic temperatures during heating, the metabolic rate of hypothyroid rats is significantly less than that of controls. The lower metabolic rate may be a reflection of the lower level of spontaneous muscular activity observed in these animals during heating. The greater tolerance of hypothyroid rats to heat thus appears to be associated with an ability to resist an increase in heat production as colonic temperature rises.