The role of triiodothyronine (T3) and T3/free thyroxine (fT4) on glucose metabolism during pregnancy: the Ma'anshan Birth Cohort Study.

Objectives Compared with other thyroid markers, fewer studies explored the associations between triiodothyronine (T3) and T3/free thyroxine (fT4) and glucose abnormality during pregnancy. Thus, we aimed to: (1) examine the associations of T3 and T3/fT4 with glucose metabolism indicators; and (2) evaluate, in the first trimester, the performance of the two markers as predictors of gestational diabetes mellitus (GDM) risk. Methods Longitudinal data from 2723 individuals, consisting of three repeated measurements of T3 and fT4, from the Man'anshan birth cohort study (MABC), China, were analyzed using a time-specific generalized estimating equation (GEE). The receiver operating characteristic curve (ROC) - area under the curve (AUC) and Hosmer-Lemeshow goodness of fit test were used to assess the discrimination and calibration of prediction models. Results T3 and T3/fT4 presented stable associations with the level of fasting glucose, glucose at 1h/2h across pregnancy. T3 and T3/fT4 in both the first and second trimesters were positively associated with the risk of GDM, with the larger magnitude of association observed in the second trimester (Odds ratio (OR) = 2.50, 95%CI = 1.95, 3.21 for T3; OR = 1.09, 95%CI = 1.07, 1.12 for T3/fT4). T3 ((AUC) = 0.726, 95%CI = 0.698, 0.754) and T3/fT4 (AUC = 0.724, 95%CI = 0.696, 0.753) in the first trimester could improve the performance of the predicting model; however, the overall performance is not good. Conclusion Significant and stable associations of T3, T3/fT4 and glucose metabolism indicators were documented. Both T3 and T3/fT4 improve the performance of the GDM predictive model.