Objective: As an essential element, selenium is very important for thyroid functions. Iodothyronine deiodinase-I is a selenium containing enzyme that is responsible for production of triiodothyronine (T3) from thyroxin (T4) in peripheral tissues. The aim of this study is to investigate the effect of different levels of selenium on thyroid hormone synthesis. Methods: 303 participants who had admitted to several clinics of Ankara Occupational Diseases Hospital and whose serum thyroid hormone and selenium levels were determined between years 2012 and 2014 were separated into 3 groups according to serum selenium levels. Group 1: 50-75μg/L, group 2: 75,1-100μg/L, group 3: 100,1-125μg/L. The thyroid stimulating hormone (TSH), free T3 (FT3) and free T4 (FT4) levels of groups were compared with each other. Results: There was statistically non-significant difference among 3 groups concerning the serum TSH and FT3 levels (p=0,91; p=0,18). Group 3 serum FT4 levels were lower than other groups. FT4 levels of group 3 were significantly different from group 1 (p= 0,005). T3/T4 ratio was significantly higher in group 3 than other groups (p=0,004). There was a negative correlation between selenium and FT4 levels (r=-0,184; p=0,001), whereas a positive correlation existed between selenium and T3/T4 ratio (r=0,178, p=0,002). Conclusion: Selenium has an important role in thyroid functions and FT4 levels increase in selenium deficiency. High FT4 levels should be considered in low selenium levels.