Early Decline in Thyroid Hormone Levels Predicts Mortality Following Congenital Heart Surgery in Neonates: A Retrospective Cohort Study

<b>Background:</b> Thyroid hormone dysregulation is a well-recognized consequence of cardiopulmonary bypass (CPB), particularly in neonates undergoing congenital heart surgery. Triiodothyronine (T3) plays a crucial role in maintaining cardiovascular stability, and an early decline in serum levels may adversely impact clinical outcomes. This study aimed to evaluate perioperative thyroid hormone changes and their association with morbidity and mortality. <b>Methods:</b> We retrospectively analyzed 132 neonates who underwent congenital cardiac surgery with CPB between January 2021 and June 2024. Serum free T3 (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) levels were measured preoperatively and within one hour after admission to the cardiac intensive care unit. Demographic, clinical, and surgical variables were recorded. Associations between thyroid hormone levels and postoperative outcomes, including in-hospital mortality, ventilation duration, vasoactive-inotropic score (VIS), and length of stay, were assessed using correlation analyses, logistic regression, and receiver operating characteristic (ROC) analysis. <b>Results:</b> Postoperatively, both FT3 and TSH levels declined significantly (<i>p</i> < 0.01), while FT4 levels remained unchanged. Lower postoperative FT3 levels were negatively correlated with prolonged invasive mechanical ventilation (rho = -0.196, <i>p</i> = 0.029) and longer hospital stay (rho = -0.183, <i>p</i> = 0.042). Overall mortality was 7.6% (<i>n</i> = 10). Non-survivors had significantly lower postoperative FT3 levels compared with survivors (<i>p</i> = 0.001). In multivariable logistic regression, postoperative FT3 was independently associated with mortality (OR = 0.22, 95% CI 0.05-1.03, <i>p</i> = 0.048). ROC analysis demonstrated good predictive performance of postoperative FT3 for mortality (AUC = 0.818), with an optimal cutoff of 2.17 pg/mL (sensitivity 72%, specificity 70%). <b>Conclusions:</b> Early postoperative suppression of FT3 is common after CPB in neonates and is independently associated with increased mortality and adverse short-term outcomes. Early assessment of thyroid function, particularly FT3, may provide valuable prognostic information and aid in risk stratification in this high-risk population.