Trimester-Specific Reference Ranges for Free Thyroxine Hormone in Iranian Pregnant Women

Physiological changes during pregnancy affect thyroid function and also free thyroxine (FT4) assay methods. These impacts make immunoassays unreliable for detecting hypothyroxinemia during pregnancy. Due to pregnant women’s adequate consumption of iodine in Iran, reference ranges of FT4 needs to be obtained for this group.

A total of 1077 Iranian pregnant women were randomly selected from health clinics in 12 provinces of Iran during all trimesters. Five hundred eighty-seven individuals were included in the study after applying the exclusion criteria to establish the reference range. Then, TSH, FT4, and TPOAB levels were examined.

The mean age of the participants was 27.6 ± 5.7 years, and the median urine iodine level was 236 μg/L (188-321). The 5th and 95th percentiles obtained for FT4 were 0.93 and 1.35 in the first trimester, 0.78 and 1.15 in the second trimester, and 0.77 and 1.15 ng/dl in the third trimester, respectively.

Due to the downward trend of the normal range of FT4 from the first to the third trimester of pregnancy, it seems necessary to consider the trimester-specific normal range for diagnosis and management of thyroid disease in Iranian pregnant women.