Selenium was first discovered by a Swedish chemist in 1817 and named after the Greek legendary goddess of the moon (Selefie). Early on, selenium was thought to be a toxic substance until 1957, when it was recognized as an essential trace element for living things. At present, the biological role of selenium is being recognized by domestic and foreign scholars. Selenium is an essential trace element that exerts its biological effects mainly through selenium-containing proteins. Selenium is involved in the basic metabolic processes of various life activities, for example, the synthesis process of DNA requires the participation of selenium-containing proteins. The appropriate nutritional status of selenium in the body is conducive to the maintenance of normal immune defense, thyroid function, reproductive function, etc. Studies have confirmed that the thyroid gland contains the highest amount of selenium among human organs, and recent studies have shown that selenium has the following important effects on iodine metabolism and thyroid function: 1, antioxidant effect: the thyroid gland produces a large amount of peroxides in the process of synthesizing thyroid hormones, and the accumulation of large amounts of peroxides in the thyroid tissue can cause damage to the thyroid follicular epithelium, causing diseases such as Hashimoto’s thyroiditis and other diseases. Selenium is the best antioxidant in human tissues and can remove excess peroxides produced during thyroid hormone synthesis, thus maintaining the balance between oxidation and antioxidation in thyroid tissue and protecting thyroid cells from oxidative stress damage. 2.Participate in thyroid hormone synthesis: Thyroid hormones are mainly divided into thyroxine (T4) and triiodothyronine (T3), of which T4 does not have hormonal activity, but only forms T3 through the action of deiodinase, while T3 has high hormonal activity and is the most important hormone to play the role of thyroid hormones. Selenium is involved in the process of converting deiodinase T4 into T3. Selenium deficiency can lead to a decrease in the activity of deiodinase, a decrease in the conversion of T4 into T3, an increase in T4 and a decrease in T3 in the blood, which affects the function of the thyroid gland. 3, selenium and thyroid autoimmunity: In recent years, the incidence of thyroid disease has increased year by year, which is not only related to the environment, mental stress, dietary structure and other factors, immune disorders also play an important role in it. Selenium has a special place in the immune system, and selenium can effectively improve the immune function of the body. When the immune disorder caused by low selenium is dominated by cytokine-mediated humoral immunity, a large number of thyroid-stimulating antibodies are produced, which promote thyroid hormone synthesis and lead to hyperthyroidism, while when cytokine-mediated cellular immunity is dominated, it causes lymphocyte infiltration in thyroid tissue, releasing cytokines and producing a large number of oxygen free radicals, which destroy thyroid follicular cells and manifest as hypothyroidism. It is also clinically proven that selenium deficiency can lead to many thyroid disorders, mainly endemic goiter, pediatric congenital hypothyroidism, Hashimoto’s goiter, and hyperthyroidism. In addition, epidemiological evidence shows that the intake of selenium is negatively correlated with the incidence of cancer, i.e., a certain amount of selenium intake can reduce the incidence of cancer. Some scholars have found that thyroid tissue in normal population contains the highest amount of selenium, while thyroid cancer patients have lower levels of selenium in their thyroid tissue, even lower than patients with other thyroid diseases. In conclusion, although there are no reports on the prevention of autoimmune thyroid disease, it has been suggested in the symposium on autoimmunity and pregnancy that selenium administration during pregnancy may prevent the development of postpartum thyroiditis. Selenium plays an important role in the maintenance of normal body functions as well as thyroid function, and selenium deficiency is associated with many thyroid disorders. However, due to the lack of specificity, serum selenium levels cannot be used as a diagnostic criterion for thyroid disorders. Given that most studies have found that serum selenium levels are significantly lower in patients with thyroid disease than in healthy populations, patients with thyroid disease can take moderate amounts of selenium supplements under the guidance of their doctors. Note that selenium supplementation requires good timing and dosage to avoid excessive intake and harm to the human body. Selenium-rich foods: foods rich in selenium include meat, especially animal liver and kidney, as well as seafood, cereals, mushrooms, onions, garlic, asparagus and other products, eggs, tuna and oysters are also rich in selenium. In the case of meeting nutritional needs, a daily supplement of 200 micrograms of selenium is appropriate