Aim: Diagnosis and treatment of thyroid disorders are essential for a healthy pregnancy. Physiologic changes that occur during pregnancy affect the functioning of the thyroid gland and thyroid function tests. The aim of this study was to determine the reference ranges of thyroid-stimulating hormone (TSH), free thyroxine (fT4), and free triiodothyronine (fT3) tests in pregnant women and to compare them with the reference ranges we use.

Materials and Methods: Between January 2022 and December 2024, 483 healthy pregnant women aged 18-45 years with thyroid hormone tests performed in our laboratory were included in the study. Healthy pregnant women without any risk factors and antithyroperoxidase positivity in all three trimesters were included in the study. TSH, fT4, and fT3 were measured in our laboratory using the electrochemiluminescence method. Reference intervals were calculated in accordance with the Clinical and Laboratory Standards Institute guidelines.

Results: 483 healthy pregnant women (221 first trimester, 130 second trimester, and 132 third trimester) were enrolled. Reference ranges for the first trimester were TSH (0.1-3.9 mIU/L), fT4 (0.9-1.5 ng/L) and fT3 (2.6-4 ); for the second trimester TSH (0.5-3.3 mIU/L), fT4 (0.7-1.3 ng/L) and fT3 (2.2-3.7 ng/L); TSH (0.5-4 mIU/L), fT4 (0.7-1.3 ng/L) and fT3 (2.3-3.5 ng/L) for the third trimester.

Conclusion: The upper reference limits of TSH, fT4, and fT3 were lower for all three trimesters according to the reference ranges we used in the laboratory. The use of appropriate trimester-specific reference ranges is significant to prevent misdiagnosis or unnecessary or inadequate treatment of thyroid dysfunction in pregnant women.