Objective: To determine the reference intervals of thyroid stimulating hormone (TSH), free T4 (fT4) and free T3 (fT3) tests from the data of our laboratory by indirect method and to compare them with the manufacturer’s reference intervals.

Material and Methods: The simultaneous TSH, fT4 and fT3 results of 195205 individuals aged 1-99 years, studied in our laboratory between January and October 2021, were included in the study. Tukey's method was used to identify and exclude outliers. Then, reference intervals of TSH, fT4 and fT3 were calculated by non-parametric and Bhattacharya methods. CLIA 19 acceptable limits were used to evaluate the compliance between reference intervals.

Results: Reference intervals for the pediatric group (1-17 years old) were TSH (0.56 - 5.93 mIU/L), fT4 (8.9 - 16.1 ng/L) and fT3 (2.74 - 5.39 ng/L) by non-parametric method; TSH (0.67 - 3.08 mIU/L), fT4 (9.2 - 16.1 ng/L) and fT3 (2.72 - 5.46 ng/L) by Bhattacharya method. Reference intervals for the adult group (18- 99 years old) were TSH (0.10 - 4.98 mIU/L), fT4 (8.7 - 16.1 ng/L) and fT3 (2.01 - 4.09 ng/L) by nonparametric method; TSH (0.18 - 3.07 mIU/L), fT4 (9.1 - 16.2 ng/L) and fT3 (2.05 - 4.13 ng/L) by Bhattacharya method.

Conclusion: Our fT4 and fT3 reference intervals were acceptable in line with manufacturer's reference intervals, but there were differences in upper reference limit of pediatric group and lower reference limit of adult group for TSH. It is so important for laboratories to determine their own reference intervals for correct interpretation of test results.