Objective: In our study, the reference intervals of serum thyroid hormones were established in 247 hospitalized preterm infants from 28 to 36 weeks of gestation at 8 - 15 postnatal days. The thyroid hormones were serum triiodothyronine (T3), free triiodothyronine (FT3), thyroxine (T4), free thyroxine (FT4), and thyrotropin (TSH). Methods: Electrochemiluminescence immunoassay was used to examine the thyroid hormone levels of serum samples from 247 preterm infants, who were grouped on sampling by gestational age. SPSS 16.0 was used to calculate the population-based reference intervals, in comparison to the manufacturer ' s suggested reference intervals. Results: Kruskal-Wallis H tests could not determine the difference in TSH levels among groups, which allowed us to develop a single interval for the study population. ANOVA determined the differences in T3, FT3, T4, and FT4 levels among groups, which allowed us to define reference intervals for preterm infants according to their gestational age. Conclusion: Developed reference intervals are useful for clinical diagnosis; however, there is a lack of consensus. These values could be used to assess the thyroid status of preterm infants and provide a foundation for clinical therapy. The results emphasized the importance of establishing gestational age-based reference intervals for the clinical laboratory.