Maternal thyroid dysfunction, especially in early pregnancy, may lead to pregnancy complications and adverse birth outcomes. Few population-based prospective studies have evaluated these effects and results are discrepant.

We examined the association of thyroid function and autoimmunity in early pregnancy with adverse pregnancy and birth outcomes.

The study used data from the prospective mother-child cohort “Rhea” study in Crete, Greece. A total of 1170 women with singleton pregnancies participated in this analysis. Maternal serum samples in the first trimester of pregnancy were tested for thyroid hormones (TSH, free T₄, and free T₃) and thyroid antibodies (thyroid peroxidase antibody and thyroglobulin antibody). Multivariable log-Poisson regression models were used adjusting for confounders.

Outcomes included gestational diabetes, gestational hypertension/preeclampsia, cesarean section, preterm delivery, low birth weight, and small-for-gestational-age neonates.

The combination of high TSH and thyroid autoimmunity in early pregnancy was associated with a 4-fold increased risk for gestational diabetes [relative risk (RR) 4.3, 95% confidence interval (CI) 2.1–8.9)] and a 3-fold increased risk for low birth weight neonates (RR 3.1, 95% CI 1.2–8.0) after adjustment for several confounders. Women positive for thyroid antibodies without elevated TSH levels in early pregnancy were at high risk for spontaneous preterm delivery (RR 1.7, 95% CI 1.1–2.8), whereas the combined effect of high TSH and positive thyroid antibodies did not show an association with preterm birth.

High TSH levels and thyroid autoimmunity in early pregnancy may detrimentally affect pregnancy and birth outcomes.