A preconception to late pregnancy community cohort study for reference values of serum thyrotropin and total thyroxine.

INTRODUCTION: Population-based preconception cohort studies to establish reference ranges for thyroid function tests during pregnancy are scarce. This study was done to establish the reference values for thyrotropin (TSH) and total thyroxine (T4) during the first and late-second trimesters in preconceptionally euthyroid, thyroid autoantibody-negative, community-dwelling pregnant women in a South-Indian district. METHODS: Newly married nulliparous women aged 18-35 years from were recruited preconceptionally using multistage cluster sampling. Baseline pre-conceptional thyroid function, including TSH, Total T4, and anti-Thyroid peroxidase(TPO) antibodies, was assessed. Women who were euthyroid and negative for TPO antibodies were included and followed throughout pregnancy, with repeat thyroid function tests during the first and second trimesters. RESULTS: Of the 1507 women enrolled, 482 pregnancies were identified during the 18-month follow up period. Among these, 182 met the inclusion criteria, with 125 assessed in the first trimester and 120 in the second trimester. The first-trimester TSH reference range was found to be 0.1-3.9 µIU/mL, and the second trimester range was 0.7-4.6 µIU/mL. Total T4 reference ranges were 5.2-17.0 µg/dL and 7.6-14.7 µg/dL, respectively. In the longitudinally studied women, TSH decreased from 2.3 ± 1.0 µIU/mL pre-pregnancy to 1.6 ± 1.0 µIU/mL in the first trimester, then rose marginally to 2.0 ± 0.7 µIU/mL in the second trimester. First-trimester TSH levels correlated positively with pre-pregnancy TSH and with abdominal circumference in these anti-TPO negative women (p=0.01). CONCLUSION: This study establishes community-based trimester-specific reference intervals for TSH and Total T4 in preconceptionally euthyroid, TPO antibody negative community dwelling Indian women.