Serum Thyroxine,Triiodothyronine,and TSH Levels in lodine-Deficient and lodine-Sufficient Rats before and after Exposure to Cold 1

Abstract

Abstract

Thyroidectomized rats are unable to survive chronic exposure to cold. Severely iodine-deficient rats, on the other hand, despite very low levels of serum T₄ (<0.5 μg/100 ml), may or may not display cold intolerance. Many of these animals (25-50%) are able to maintain their body temperatures and to survive exposure to 4°. We had previously shown that serum T₃ levels are relatively well maintained in severely iodine-deficient rats, and in the present investigation we sought to determine whether survival of iodine-deficient rats in the cold could be correlated with serum T₃ levels, either initially or in response to the cold exposure. In two separate experiments we observed no correlation between initial serum T₃ levels and survival in the cold. The cold-sensitive rats showed slightly lower serum T₃ levels than the cold-resistant rats after cold exposure, but the difference was not statistically significant. Our data favor the view that thyroid hormone plays only a permissive role in the increased thermogenesis elicited in rats by chronic exposure to cold.