Selenium, one of the essential trace minerals, is present in vivo in form of selenoproteins. Iodothyronine deiodinase, a selenoprotein, is involved in the activation and inactivation of thyroid hormone. Therefore, patients with selenium deficiency may present changes in thyroid hormone levels due to inhibition of T4 to T3 conversion; however, this assumption is still under debate. In the present study, we retrospectively investigated the thyroid function in 22 patients with selenium deficiency. Thyroid stimulating hormone (TSH) and free T4 (FT4) levels were increased in 3 (14%) and 5 (23%) patients, respectively, and free T3 (FT3) levels were decreased in 6 (27%) patients. The FT4/FT3 ratio was significantly higher in patients with selenium deficiency than that in the control group. There appeared to be a positive correlation between the decreased rate of selenium levels and FT4/FT3 ratio, thereby indicating that patients with severe selenium deficiency also exhibited abnormal thyroid hormone levels. Furthermore, when selenium was supplemented in seven patients with abnormal thyroid hormone levels, the TSH, FT4, and FT4/FT3 ratio were significantly decreased and FT3 levels were increased. Collectively, patients with selenium deficiency could present the characteristics of not only low FT3 but also high FT4 and FT4/FT3 ratio.