Thyroid state and working for heat in the cold

Abstract

Four experiments were conducted in which rats in a cold room were allowed to obtain a burst of heat from a heat lamp by pressing a lever. When working at 2°C, hypothyroid rats began to work for heat at a steady rate earlier in a 16-hour session than did euthyroid rats. This rate itself was both higher and more steady for hypothyroid than for euthyroid rats. Euthyroid and hypothyroid rats working at 5°C showed a difference only in time required to attain their steady rate. In one experiment, the chronic administration of l-triiodothyronine to hypothyroid rats led to a significant decrease of lever presses as compared to the performance of normal rats. Discontinuance of l-triiodothyronine led to a gradual recovery of the high rate. The differences in this kind of behavior between hypothyroid and euthyroid animals were attributed to differences in drive state arising from the tendency of body temperature of hypothyroid animals to decline more rapidly in the cold.