Maternal-fetal Morbidity and Mortality in the pregnant population with thyroid dysfunction in Catalonia

Abstract Background Thyroid dysfunction (TD) during pregnancy has been associated with adverse maternal and perinatal outcomes, although evidence remains inconsistent. The physiological changes of gestation and variability in laboratory reference ranges complicate diagnosis and management. This study aimed to determine the prevalence of maternal–fetal morbidity and mortality associated with TD in Catalonia and to evaluate whether first-trimester TSH monitoring influences these outcomes. Methods A population-based descriptive study was conducted using the SIDIAP database, including 111,811 pregnancies between 2014 and 2016. TD was classified as hyperthyroidism, untreated hypothyroidism, treated hypothyroidism, or no TD. Outcomes included miscarriage, preeclampsia, prematurity, cesarean section, low birth weight, APGAR  4 mIU/L, or not measured. Multivariable logistic regression assessed miscarriage risk adjusting for age, obesity, and TD. Results TD prevalence was 5.6%. Treated hypothyroidism was associated with higher rates of preeclampsia (5.7%), cesarean section (24.8%), and prematurity (6.8%). Untreated hypothyroidism showed increased preeclampsia (6.1%). Miscarriage risk increased progressively with rising TSH values, independently of TD diagnosis. Hyperthyroidism remained associated with miscarriage independently of TSH values. Lack of first-trimester TSH testing was consistently associated with higher miscarriage rates in all TD groups (treated: 11.4%; untreated: 8.2%). Among women without TD, miscarriage increased with rising TSH values, suggesting undiagnosed dysfunction. No significant differences were observed in perinatal mortality or low birth weight. Conclusions TD during pregnancy increases the risk of preeclampsia, prematurity, and cesarean section. Miscarriage risk is primarily driven by inadequate TSH control rather than TD diagnosis itself. The absence of first-trimester TSH testing is associated with worse outcomes. More than 6% of women without known TD had TSH > 4 mIU/L, supporting consideration of universal thyroid screening during pregnancy.