- OBJECTIVE: To investigate pregnancy outcome in women with hypothyroidism. STUDY DESIGN: A population-based study was performed comparing all singleton pregnancies of patients with and without hypothyroidism. Hypothyroidism was diagnosed and treated before pregnancy. Patients lacking prenatal care were excluded from the analysis. Deliveries occurred between the years 1998 and 2002 in a tertiary medical center. Stratified analysis using a multiple logistic regression model was performed to control for confounders. RESULTS: During the study period 139,168 singleton deliveries occurred at our medical center. Of those, 0.8% (n = 1,102) were in patients with hypothyroidism. Using multivariate analysis with back-step elimination, the following risk factors were significantly associated with hypothyroidism: fertility treatments, recurrent abortions, diabetes mellitus, previous cesarean section and advanced maternal age. No significant differences regarding pregnancy complications, such as placental abruption, preterm deliveries or postpartum hemorrhage, were noted between the groups. However, patients with hypothyroidism had higher rates of cesarean deliveries (20.1% vs. 11.5%, p<0.001). This association remained significant even after controlling for confounders, such as diabetes mellitus, previous cesarean section, fertility treatments, recurrent abortions and advanced maternal age using a multivariate analysis. Perinatal outcomes, including birth weight <2,500 g (10.4% in the hypothyroidism group vs. 9.5% in the comparison group, p=0.159), Apgar score <7 at 5 minutes (0.8% vs. 0.6%, p=0.312) and perinatal mortality (1.4% vs. 1.3%, p=0.950) did not differ between the groups. Nevertheless, patients with treated hypothyroidism and adverse perinatal outcomes had well-controlled disease, using thyroxine doses of 50-100 μg daily, with mean thyroid stimulating hormone levels of 3.035 ± 2.4 mU/L. CONCLUSION: Treated maternal hypothyroidism is not associated with adverse perinatal outcome. However, hypothyroidism is an independent risk factor for cesarean section.