**4. Selenium**

Selenium is a trace element that plays an essential role in thyroid hormone synthesis, because two enzymes involved in thyroid hormone production are selenoproteins: the deiodinases and glutathione peroxidase. Selenium influences the immune system probably by enhancing plasma glutathione peroxidase and thioredoxin reductase activity and by decreasing toxic concentrations of hydrogen peroxide and lipid hydroperoxides, resulting from thyroid hormone synthesis (19,20). A deficit of selenium results in increased intrathyroidal levels of hydrogen peroxide, which possibly increase the activity and immunogenicity of Thyroid Peroxidase (TPO) (21). Low selenium blood levels cause increased thyroid volume and thyroid hypoechogenicity, a marker for lymphocytic infiltration (22).
