Reference Intervals of Thyroid Function Tests in First Trimester Vietnamese Pregnant Women

Although TSH suppression by elevated β-hCG is essentially seen during first trimester, differences in TSH reference ranges between various countries have been reported. Physiologic changes during pregnancy may also influence FT4 assays. This study aims to establish method-specific reference intervals (RIs) of TSH, FT4, and FT3 in Vietnamese, first trimester pregnant women.

This cross-sectional study was conducted at My Duc Hospital, Ho Chi Minh, Vietnam. Women with singleton pregnancies in the first trimester and conceived naturally were included. Those with a history of thyroid disease, positive thyroid-specific autoantibodies, diffuse goiter or one thyroid nodule > 10 mm in size or ≥ 2 nodules detected by ultrasound, and taking medications affecting thyroid function were excluded. Serum TSH, FT4, and FT3 were measured by chemiluminescent detection technology on the Access 2 Immunoassay System (Beckman Coulter, Inc., USA). Intra- and interassay coefficients of variations (CV) were 3.6% and 4.4% for TSH, 5.4% and 6.1% for FT4, 6.6%, and 6.0% for FT3, respectively. The 2.5th and 97.5th percentiles were used to determine RIs.

Between August 1, 2017, to December 1, 2018, there were 876 pregnant women who fulfilled inclusion and exclusion criteria. They had a mean age of 30.1 years, an average BMI of 21.3 kg/m2, and 77.3% of them were primigravida. The RIs for TSH, FT4 and FT3 were 0.17 - 2.35 mIU/L, 0.67 - 1.11 ng/dL and 2.82 - 3.90 pg/mL, respectively.

Established RIs for TSH, FT4, and FT3 in Vietnamese women would help to reduce the misdiagnosis of gestational thyroid disorders.