Selenium requirements of rats for normal hepatic and thyroidal 5'-deiodinase (type I) activities

The nutritional requirement of selenium for type I 5'-deiodinase activity in thyroid compared with liver was assessed in rats. Male weanling Sprague-Dawley rats were fed a torula yeast-based diet for 20 wk. One group of rats was fed the Se-deficient basal diet (0.01 mg Se/kg). The other three groups were fed the basal diet plus sodium selenite at 0.05, 0.1 and 0.5 mg Se/kg diet. Liver 5'-deiodinase and glutathione peroxidase (GSH-Px) activities were depressed in the group fed the Se-deficient (basal) diet compared with the other groups. Liver 5'-deiodinase activity in the group fed 0.05 mg Se/kg diet was as high as in the groups fed 0.1 and 0.5 mg Se/kg diet, whereas GSH-Px activities in the groups fed 0.05 and 0.1 mg Se/kg diet were intermediate in value. Feeding the Se-deficient diet for 20 wk did not cause a suppression in 5'-deiodinase in the thyroid, and thyroid GSH-Px activity was approximately 40% of that in the other groups. In rats fed Se-supplemented diets, thyroid GSH-Px was approximately 20% or less of the activity found in liver. Plasma thyroxine was higher in the group fed the Se-deficient (basal) diet, but there were no differences in plasma 3,3',5-triiodothyronine among all groups. The results suggest that the nutritional Se requirement for 5'-deiodinase is less than that for GSH-Px and is approximately 0.05 mg Se/kg in the diet for normal activity in the liver and approximately 0.01 mg Se/kg for normal activity in the thyroid. Thyroid seems to be a priority organ over liver for Se when the intake of the element is limited.