Objective To explore the risk of different levels of pre-pregnancy obesity on trimester-specific thyroid dysfunction.
Methods Questionnaire information， blood samples， and urine samples from a 2017 pregnancy cohort study in Shanghai， China were collected. A total of 2 455 pregnant women were included in the analysis. Pre-pregnancy BMI was calculated based on the height and self-reported pre-pregnancy weight. Serum TSH， total thyroxine （TT4）， free thyroxine （FT4）， total triiodothyronine （TT3）， free triiodothyronine （FT3）， thyroid globulin antibody（TgAb）， and Thyroid peroxidase antibody （TPOAb） were measured using the electrochemiluminescence method. Urine iodine levels were measured using the acid digestion method. Levels of thyroid function indexes of pregnant women with different degrees of obesity during pre-pregnancy were compared， and trimester-specific thyroid dysfunction was evaluated according to the reference range of trimester-specific thyroid hormone established by this cohort. Multivariate logistic regressions analysis was used to assess the correlation between pre-pregnancy obesity and trimester-specific thyroid dysfunction.
Results As the degree of obesity increased， maternal levels of FT3 and TT3 gradually increased during pregnancy （P<0.001， P=0.001）， while FT4 levels gradually decreased （P=0.001）. Multivariate logistic regression analysis showed that compared with the normal weight group， pregnant women who were overweight or obesity before pregnancy had a significantly higher risk of hypothyroxinemia （OR=3.85， 95%CI： 2.08‒7.14， P<0.001） and high TT3 （OR=2.78， 95%CI： 1.45‒5.26， P=0.002） during pregnancy.
Conclusion Pre-pregnancy overweight or obesity can increase the risk of thyroid dysfunction during pregnancy.
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