Thyroxine metabolism and thyroid function in the cold-adapted rat.

Abstract

Thyroid function and thyroxine (T4) metabolism were assessed in rats maintained at 4 or 22 C for at least 3 months. In the coldadapted rat, thyroidal uptake of iodine was increased up to 3-fold. Since total glandular iodine was unchanged, thyroid secretion rate was presumed to be similarly increased. The metabolism of T4 was studied in rats (KC104-treated to prevent 131I uptake by the thyroid) iso topically equilibrated with 131I-T4 of known specific activity. Cold-adapted rats excreted less iodide in urine than did controls, when receiving twice the dose of T4, and not significantly more than controls when given 4 times the dose, although the latter dose raised the serum T4 level above normal. The apparent reduction in the rate of deiodination was not due to changes in the intrinsic deiodinating activity of tissues, or to changes in the T4-binding activity of serum. Total and free T4 levels were not altered. The total turnover of T4 was increased due to a marked increase in the clearance from plasma thro...