Investigating Factors Associated with Thyroid Hormone Measurements During Pregnancy

Background Accurate assessment of thyroid status is essential for maternal and fetal management during pregnancy. This study measured human chorionic gonadotropin (HCG), albumin, and thyroxine-binding globulin (TBG) levels during pregnancy to clarify how their fluctuations affect thyroid hormone measurements by two immunoassays—chemiluminescent immunoassay (CLIA) and electrochemiluminescence immunoassay (ECLIA). Method Free thyroxine (FT4), free triiodothyronine (FT3) and thyrotropin (TSH) levels were measured in 897 serum samples obtained from 604 pregnant women by two immunoassays. In 176 cases selected from each trimester, thyroid hormone concentrations were also measured by liquid chromatography–tandem mass spectrometry (LC-MS/MS) following ultrafiltration. Associations between thyroid function and relevant determinants were evaluated using multivariable regression analysis. Results Throughout pregnancy, 64 samples (7.13%) measured by CLIA and 241 samples (26.87%) measured by ECLIA had TSH concentrations less than 0.01 mIU/L. The upper limits of thyroid hormone concentrations were observed between 8 to 15 weeks of gestation. In late pregnancy, the lower limits of thyroid hormone concentrations determined by the immunoassays fell below the lower limits of the nonpregnant reference intervals. Thyroid hormone concentrations measured by immunoassay correlated significantly with LC-MS/MS concentrations. In multivariable regression analysis, only HCG was significantly associated with immunoassay measurements of thyroid hormones. Under conditions of TBG ≥31 μg/mL, women with albumin <3.8 g/dL had lower thyroid hormone concentrations than those with ≥3.8 g/dL. Conclusion Gestational thyroid hormone concentrations appear to be influenced by HCG levels. As with LC-MS/MS use, immunoassay measurements may vary with albumin and TBG concentrations. These findings underscore the need to consider such fluctuations when interpreting thyroid function tests in pregnant women.