Aim: To establish trimester and population based laboratory-specific rational reference ranges with multiples of median (MoM) values for thyroid hormones. Materials and Methods: The study was conducted in the obstetrics outpatient clinic of a tertiary health care center between April, 2021 and August, 2021. Healthy pregnant women without any risk factors and antithyroperoxidase positivity were recruited for all three trimesters. Serum thyroid-stimulating hormone (TSH), free thyroxine (FT4) and free triiodo-thyronine (FT3) levels were measured by the electrochemiluminescence technique. Trimester specific reference ranges were determined with 5. and 95. percentiles of TSH, FT3 and FT4 values with MoM values. Results: Overall, 484 healthy pregnant women, including 140 women in their first trimester, 204 women in their second trimester and 140 women in their third trimester were recruited. In the first trimester, 90% (n=126) women’s TSH levels were within the reference limits (0.19-3.25 uIU/mL and 0.19-3.25 MoM) and 11.1% (n=14) had TSH levels above 2.5 uIU/L. For second trimester, 90.2% (n=184) had TSH levels within the reference limits (0.65-3.83 uIU/mL and 0.4-2.36 MoM) and 8.15% (n=15) had TSH levels over 3 uIU/L. In the third trimester, 90.7% (n=127) women’s TSH levels were within reference limits (0.62-3.78 uIU/mL and 0.39-2.35 MoM) and 3.15% (n=4) had TSH levels over 3 uIU/L. Conclusion: Accurate diagnosis and management of thyroid disease for pregnant women is crucial for maternal-fetal outcomes. Established reference ranges for three trimesters in this study were all higher than the recommended fixed ranges of 2011 American Thyroid Association guideline.