A Prospective Study to Determine Trimester-Specific Thyroid Function Reference Intervals

Precise thyroid function assessment during pregnancy prevents inaccurate diagnosis of thyroid disease. This study aimed to establish trimester-specific reference intervals for thyroid function tests (TFTs) in Taiwanese pregnant women and analyze thyroid hormone and anti-thyroid antibody changes through gestation. This prospective study at Taipei Veterans General Hospital (2019-2022) included 209 pregnant women. After excluding those with positive anti-thyroid antibodies, use of thyroid-related medications, or plans for abortion, 123 participants were eligible for thyroid function reference interval analysis. TFTs including thyroid-stimulating hormone (TSH), free T4, total T4, free T3, and total T3 along with antibodies were measured once per trimester using a Roche electrochemiluminescence immunoassay. One spot urine sample from each woman was collected at baseline to ensure iodine adequacy and urinary iodine concentration was measured by inductively coupled mass spectrometry. The median urinary iodine concentration was 162.5 µg/L, indicating that the analyzed population had sufficient iodine status. TSH levels were significantly lower in the first trimester (0.007-3.08 µIU/mL) compared to the second (0.33-4.55 µIU/mL) and third trimesters (0.29-4.64 µIU/mL) (p < 0.001). Levels of free T4 and free T3 decreased consistently throughout pregnancy (p < 0.001), while total T3 levels remained stable. In contrast, total T4 levels, anti-thyroid peroxidase antibody titers, and the proportion of women with positive anti-thyrotropin receptor antibodies were higher in the first trimester compared to the second and third trimesters. This study underscores the importance of trimester-specific, population-based reference intervals for TFTs using the Roche assay and represents the first investigation in Taiwanese populations.