Peripheral metabolism of thyroid hormones in vitamin A-deficient rats.

Some aspects of thyroid hormone metabolism were studied in rats on a vitamin A-deficient diet for 7-8 weeks. It was shown that, in deficiency, thyroxinemia, the extrathyroidal thyroxine (T4) pool and the T4 biological half-life were increased, and the T4 distribution space and the T4 disposal rate were similar to those in control rats. The in vitro formation of triiodothyronine (T3) from T4 by liver microsomes was slightly decreased under basal conditions but similar after addition of glutathione. So the increased T4 half-life in deficient animals may be related to a decreased 5'-monodeiodase activity resulting from decreased glutathione availability. In deficient rats, the T3 serum level was increased, the T3 biological half-life was similar and the T3 distribution space was much decreased.