Selenium is an important element in maintaining the physiological balance of the human body and it influences the immune response, cell growth and viral defense. However its most important role lies in the maintenance of normal thyroid function. In total, there are 25 selenoproteins that play various roles in the body. Selenium is an important molecule in the active sites of glutathione peroxidase, deiodinase and sulfur oxidoreductase protein reductase. Thus, selenium has the most important role in thyroid hormone synthesis and function, protection of thyroid cells from free radicals and oxidative damage. To maintain the activity of selenium-containing proteases daily intake of selenium is about 60-75 ug. Selenium deficiency decreases the activity of glutathione peroxidase, which leads to oxidative damage, and deiodinase, which damages the thyroid gland. More importantly, selenium deficiency causes thyroid autoimmune response and is an important substance involved in thyroid autoimmune response to inflammation and primary hyperthyroidism. Because of cell cycle regulation, selenium deficiency may play a driving role in the development of thyroid cancer.