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

Abstract

Purpose: 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.

Methods: 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.

Results: 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.

Conclusions: 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.