Trimester-specific reference intervals for thyroid function parameters in Indian pregnant women during final phase of transition to iodine sufficiency

BACKGROUND: Interpretation of thyroid function tests during pregnancy depends on gestational age, method, and population-specific reference intervals. Therefore, there is a worldwide trend to establish trimester-specific levels for different populations. The aim of this study was to establish a trimester-specific reference range for thyroid function parameters during pregnancy in Indian women. MATERIALS AND METHODS: percentile values were used to determine the reference ranges for thyrotropin (TSH), free thyroxine (FT4), total thyroxine (TT4), and total triiodothyronine (TT3) for each trimester of pregnancy. RESULTS: The reference range for TSH for first trimester was 0.19-4.34 μIU/ml, for second trimester 0.46-4.57 μIU/ml, and for third trimester 0.61-4.62 μIU/ml. The reference range during three trimesters for FT4 (ng/dl) was 0.88-1.32, 0.89-1.60, and 0.87-1.54, for total T4 (μg/dl) was 5.9-12.9, 7.4-15.2, and 7.9-14.9. In nonpregnant women, FT4 was 0.83-1.34, total T4 was 5.3-11.8, and TSH was 0.79-4.29. The mean UIC in nonpregnant women was 176 ± 15.7 μg/L suggesting iodine-sufficiency in the cohort. CONCLUSION: The trimester-specific TSH range in pregnant women in this study is not significantly different from nonpregnant reference range in the final phase of transition to iodine sufficiency in India.