Article ID: EJ25-0423
Hypothyroidism is reported to associate with adverse pregnancy outcomes. However, there are few reports in iodine-sufficient or -excess areas. We investigated the association of hypothyroidism in early pregnancy with pregnancy outcomes in an iodine-rich area, Japan. We conducted a retrospective cohort study at a tertiary center between 2004 to 2013. The eligible participants were classified into three groups: euthyroidism, subclinical hypothyroidism (SCH), or overt hypothyroidism (OH) groups, based on their thyroid hormone levels in the first trimester of pregnancy (median 11.6 weeks, IQR 10.6–12.6). SCH and OH were defined as elevated TSH levels with normal and low FT4 levels, respectively, using cohort-specific reference ranges in early pregnancy. The primary outcomes were miscarriage, preterm birth, small for gestational age (SGA), and their composite. Multivariate logistic regression analyses and restricted cubic spline analyses fitted to the regression models were performed to analyze the association and dose-dependency. 5,366 pregnant women were classified as euthyroidism, 143 as SCH, and 21 as OH. The composite outcome was not associated with SCH (odds ratio [OR], 0.73; 95%CI, 0.41–1.31), but significantly associated with OH (OR, 3.43; 95%CI, 1.36–8.68). Preterm birth was not associated with SCH (OR, 0.69; 95%CI, 0.28–1.69), but significantly associated with OH (OR, 5.56; 95%CI, 1.98–15.63). In this Japanese single-center cohort, SCH diagnosed in the first trimester was not associated with selected adverse pregnancy outcomes; however, miscarriage risk in early pregnancy could not be adequately assessed. The restricted cubic spline models for the primary outcomes showed J-shaped association with TSH, while the dose–response analysis did not show a clear increase in risk within the reference range of FT4. SCH in early pregnancy was indicated to have no association with the adverse pregnancy outcomes in an iodine-rich area. The risk of adverse pregnancy outcomes was increased exclusively in the low FT4 range.
