Establishing Reference Intervals for Serum Thyroid Stimulating Hormone and Free Thyroxine in the First Trimester of Pregnancy in Vietnam

Introduction: Trimester-specific thyroid reference intervals are essential, especially in the first trimester, but remain limited in Vietnam. This study aimed to establish reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) in the late first trimester of pregnancy, and to evaluate their associations with maternal characteristics. Methods: A cross-sectional study was conducted among healthy pregnant women between 11 weeks 0 days and 13 weeks 6 days at Hung Vuong Hospital, Ho Chi Minh City. Serum TSH and FT4 were measured by electrochemiluminescence immunoassay (Cobas e801, Roche Diagnostics). Data were analyzed using SPSS 27.0. Reference intervals were calculated non-parametrically (2.5th–97.5th percentiles) according to CLSI EP28-A3c, after outlier exclusion using the interquartile range (IQR) method. Results: The late first-trimester reference intervals (2.5th–97.5th percentiles) were 0.20–2.86 µIU/mL for TSH (90% CI: 0.17–0.28 and 2.68-3.13) and 0.83–1.39 ng/dL for FT4 (90% CI: 0.81–0.86 and 1.35-1.60). Multiple linear regression showed no significant associations between TSH and maternal age, gestational age, or BMI. FT4 exhibited a weak inverse association with BMI (β = −0.016, p < 0.001) with low predictive value and no clinical significance, and was not significantly associated with other variables. Conclusions: This study established late first-trimester reference intervals for TSH and FT4 in healthy Vietnamese pregnant women between 11 weeks 0 days and 13 weeks 6 days of gestation, with minimal influence of maternal characteristics. These findings support population- and method-specific thyroid reference intervals in pregnancy.