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 (Ta) in a temperature gradient, core temperature (Tc), 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 degree, whereas selected Ta 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.