Significance of selenium in thyroid physiology and pathology.

Selenium is pivotal element in maintaining homeostasis of human body. It is capable of exerting an influence on immunological responses, cell growth and viral defence. Nevertheless, it is mostly required for the proper thyroid function. There were described 25 selenoproteins, which play various roles in human body. Selenium is an essential particle in the active site of enzymes such as GPXs (glutathione peroxidases), Ds (deiodinases) and TRs (thioredoxin reductases). Owing to this, it has a fundamental importance in the synthesis and function of thyroid hormones, and protects cells against free radicals and oxidative damage. Intake of selenium necessary to maintain suitable selenoenzyme activity ranges from 60 μg to 75 μg per day. Selenium deficiency contributes to decreased activity of GPXs, which can lead to oxidative damage, or Ds, which is connected with impaired thyroid activity. Moreover, a low selenium concentration causes autoimmune processes in the thyroid gland, thus selenium deficiency is essential in pathogenesis of autoimmune thyroiditis or Graves' disease. Because of regulation of the cell cycle, a decreased concentration of selenium impacts on the development of thyroid cancer.