Behavioral and autonomic thermoregulation in the rat following propylthiouracil-induced hypothyroidism

A reduced body temperature is a common symptom of hypothyroidism and may result from a deficiency in metabolic heat production. However, a reduced metabolism does not necessarily imply a failure in thermoregulatory control if other thermoeffectors, in particular behavioral thermoregulation, are operative. To address this issue, selected ambient temperature (T-a) in a temperature gradient, core temperature (T-c), heart rate (HR), and motor activity (MA) were monitored via radiotelemetry in euthyroid rats and rats made hypothyroid by the administration of 0.05 mg/ml propylthiouracil (PTU) in drinking water for approximately 15 days. Core temperature of PTU-treated rats was reduced by 0.3 degrees whereas selected T-a was increased by 2.3 degrees. PTU treatment led to significant reductions in HR, whereas MA was unaffected. Thermoregulatory behavior did not reverse the PTU-induced hypothermia, suggesting that PTU-induced hypothyroidism leads to a regulated reduction in body temperature (i.e., decrease in the set point). A reduced set point seems to be an adaptive response that lowers the metabolic requirements for thermoregulation in the hypothyroid rat. (C) 1997 Elsevier Science Inc.