ROLE OF THE THYROID IN METABOLIC RESPONSES TO A COLD ENVIRONMENT

Abstract

When adult rats were exposed to a low environmental temp. (1.5[degree][plus or minus]1[degree]C ) an increase in MR1.5[degree] (total metabolic rate measured at 1.5[degree]C ) took place immediately and an increase in MR30[degree] (metabolic rate measured at 30[degree]C) occurred gradually reaching a max. in about 2 weeks. The high levels of MR30 and MR30[degree] were main-tained for a period of at least 3 mos., the period of observation. MR1.5[degree] was greatly reduced when the O2 consumption was measured under Na-pentobarbital anesthesia, but anesthesia did not cause a reduction in MR30[degree] to as low a level as, or by a greater percentage than in normal controls. In "athyroid" rats (thyroidectomy or propylthiouracil) which received a constant dose of 2.5-5 [mu]g. of thyroxine daily a marked increase in MR30[degree] occurred during exposure to cold. Thus the elevation of metabolic rate in cold is not dependent on increased activity of the thyroid. Although admn. of NaI decreased the degree of thyroid hyperplasia in cold it did not affect the elevated MR30[degree]. Rats acclimatized to the low temp. lived long enough after deprivation of thyroid function for studies on metabolic rate to be carried out. The MR1-5[degree] was elevated but the MR30[degree] of these athyroid rats approached the low values observed in athyroid rats living at room temp. An increase in thyrotropic activity of the pituitaries was found after 2 weeks of exposure to cold, and this increase was sustained during a period of 3 months of observation. The enhanced thyrotropic activity was not affected by the admn. of NaI. It is postulated that the increase in metabolic rate (MR30[degree]) in animals after exposure to cold does not depend on a hyperthyroid state but does depend on the presence of thyroid hormone and that this increase in metabolic rate is associated with the ability of the animal to survive.