Metabolic alterations induced by chronic heat exposure in the rat: the involvement of thyroid function

The effects of chronic exposure to high environmental temperature (34°C) on T₄ production rate, foodintake, growth-rate and resting metabolic rate were investigated in adult male rats. This study was designed to examine the extent of variations and possible relationships between these parameters. As compared to control rats of the same body weight kept at 25°C, rats exposed to 34°C for 3–4 weeks exhibited a retarded growth-rate: 2.3 vs 4.0 g/day, a reduced food-intake: 15.2 vs 23.2 g/day, a decreased T₄ production-rate: 1.8 vs 2.7 g/day and a decreased oxygen consumption: 4.0 vs 5.4 ml/min. Heat-exposure altered the 4 parameters to a similar extent. T₄ supplementation (3 g/day) which induced a decrease in plasma TSH concentration, did not restore a normal growth-rate in heat-exposed rats. The decreased food-intake of the heat-exposed rats was not associated with any significant changes in the daily pattern of variations of liver glycogen content, or in the mean daily levels of blood glucose or insulin. The ratio T₃ to rT₃ in plasma was not altered by chronic heat exposure. When rats which had been chronically exposed to heat (25 days at 34°C) were exposed to 25°C, growth-rate, food-intake and oxygen consumption rapidly increased to control values whereas the rate of T₄ production remained low. It is concluded that (1) a decrease in thyroid hormone economy is not directly involved in the alterations of growth and energy expenditure in rats chronically exposed to heat, (2) heat exposure does not lead to the establishment of a fasted state resulting from a large reduction in voluntary food intake, (3) metabolic alterations induced by heat exposure are rapidly and completely reversible upon decreasing the environmental temperature.