Thioredoxin reductase reduces oxidized forms of vitamin C. Dehydroascorbic acid (DHA) is an oxidized form of ascorbic acid which is actively imported into cells by GLUT1 which is a glucose transporter. Selenium can result in increased activity of thioredoxin reductase. Increased activity of thioredoxin reductase will result in reduction of dehydroascorbic acid to ascorbic acid which is not transported by GLUT1. Dosages of selenium more than 200-300 micrograms a day could result in vitamin C deficiencies in cells via heightened plasma reduction of dehydroascorbic acid.to ascorbic acid. Selenium should never be taken at the same time as vitamin C.