While large doses of selenium are toxic, trace amounts are important for promoting blood sugar metabolism and synthesizing antioxidants such as glutathione peroxidase and thioredoxin reductase, which help neutralize free radicals produced naturally during metabolism. Chinese researchers found a link between low blood selenium levels and an increased risk of thyroid disease.  They examined more than 6,000 people living in two areas of China. People with low blood selenium showed an increased risk of low thyroid output, immune system-linked thyroid disease and enlarged thyroid. Increased selenium intake through foods or supplement might prevent many cases of thyroid disease. Natural food sources of selenium include nuts, cereals and mushrooms. Selenium is included in most multivitamins.