Reference Intervals for Thyroid Function and the Negative Correlation between FT4 and HbA1c in Pregnant Women of West China

Background: It is necessary to establish the reference intervals of thyroid hormones in the three trimesters and postpartum period of healthy pregnant women in western China. Thyroid hormones play important roles in metabolic processes. Methods: The concentrations of TPOAbs, TSH, and FT4 were measured by an ADVIA Centaur 2000 and HbA1c in the second trimester was measured by BIO-RAD D-10. Results: 3435 pregnant women and 834 non-pregnant women were selected to establish the reference intervals. The level of TSH was the lowest (from 0.051 to 4.489 mIU/L) in the first trimester (p < 0.001) and FT4 was lower in the second (from 10.97 to 15.49 pmol/L) or third trimester (from 9.49 to 16.25 pmol/L) compared with the other periods (p < 0.001). 20% of participants had TSH levels below the lower limit of the manufacturer's reference interval (0.55 mIU/L) in the first trimester. 6.9% in the second and 16.1% of participants in the third trimester had FT4 levels below the lower limit of the manufacturer's reference interval. In multiple regression analyses, FT4 was independently associated with HbA1c (OR = 0.191; 95% CI 0.052, 0.698; p = 0.012), BMI (OR = 0.697; 95% CI 0.525, 0.927; p = 0.013), and TSH (OR = 0.647; 95% CI 0.428, 0.978; p = 0.039). Nonpredictive variable was age. Conclusions: The established reference intervals of TSH and FT4 reflected the changes of thyroid hormones veritably during gestation, and concentrations of FT4 in the first trimester were negatively correlated with HbA1c in the second trimester.