Thyroid Hormone Reference Ranges in Iodine-Deficient Pregnant Women: An Analysis in Foothills of Himalaya, Uttarakhand

Background: Thyroid dysfunction during pregnancy poses significant maternal and fetal health risks, particularly in iodine-deficient regions like the Himalayan foothills. Population-specific thyroid hormone reference ranges are essential for accurate diagnosis, yet limited data exists for iodine-deficient pregnant populations in India. Aim & objective: To establish trimester-specific serum TSH reference ranges for pregnant women in the iodine-deficient Himalayan foothill districts of Uttarakhand. Methodology: This cross-sectional observational study was conducted at three major districts of Uttarakhand i.e. Dehradun, Tehri Garhwal and Pauri Garhwal from March 2020 to August 2023. A total of 544 pregnant women (181 first trimester, 181 second trimester, 182 third trimester) were recruited using systematic random sampling. Trimester-specific reference ranges were calculated using 2.5th and 97.5th percentiles. In addition to TSH, urinary iodine excretion (UIE) was measured to characterize iodine status; median UIE values were 148, 155, and 147 μg/L in the first, second, and third trimesters respectively, indicating borderline-to-mild iodine deficiency per WHO 2007 criteria (adequate threshold: ≥150 μg/L). Free T4 (fT4) was also measured with trimester-specific ranges of 0.47–3.56, 0.39–3.67, and 0.34–4.50 ng/dL respectively. Total T4 (TT4) was not assessed. Results: Mean TSH levels increased progressively across trimesters (2.18±1.34 to 3.42±1.89 mIU/L, p<0.001). Established reference ranges showed substantially higher TSH upper limits compared to international guidelines: first trimester (0.12-4.87 vs 0.1-2.5 mIU/L), representing 94.8% difference. Using international ranges, 50.2% of participants would be classified as having thyroid dysfunction compared to 30.3% using population-specific ranges, indicating potential 19.9 misclassification. Conclusion: This study establishes the first trimester-specific thyroid hormone reference ranges for pregnant women in the iodine-deficient Himalayan foothill region. The findings demonstrate significant differences from international standards, highlighting the critical importance of population-specific reference ranges to prevent misdiagnosis and inappropriate treatment in iodine-deficient populations.