Selenium, a non-metal chemical element, is present in the human body in trace quantities and accumulates mostly in the thyroid gland. Selenium-rich food includes offal, meat and meat products, seafood, milk and dairy products, yeast, and bread. The aim of the study is to collate and discuss research results obtained over the last 13 years regarding the influence of selenium in diseases of the thyroid gland. Selenium deficiency can lead to a number of thyroid diseases. Over 30 proteins containing selenium called selenoproteins, and their numerous functions in the human body, have been identified. Research shows that selenium is essential for proper synthesis, activation and metabolism of thyroid hormones. It also can postpone the development of hypothyroidism, reduce the level of anti-thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb), normalize thyroid echogenicity in ultrasound examination, improve the condition of patients with Graves' disease andautoimmune thyroiditis (AIT) (Hashimoto's thyroiditis), and reduce the incidence of postpartum thyroid dysfunction. The recommended dose of selenium for adults is 45-55 mu g 24 h(-1) (Polish recommendations) or 55 mu g 24 h(-1) (recommendations in the US). Selenium may have a toxic effect in doses above 400 mu g 24 h(-1); therefore a threshold of 400 mu g 24 h(-1) has been adopted as the maximum safe dose. The role of selenium supplementation in diseases of the thyroid gland is still subject to discussion.