Background: Congenital cardiovascular defects, or congenital heart defects, are structural abnormalities that result from improper development of the heart or major blood vessels. Aim: To detect thyroid dysfunction in children with CHD undergoing open heart surgeries on cardiopulmonary bypass and effect of thyroid dysfunction on postoperative ICU stay, period of mechanical ventilation, morbidity and mortality. Patients and methods: This was a Cross-sectional analytic study performed on fifty patients with congenital heart defects, 25 of them suffering from Cyanotic heart defects and 25 of them suffering from Acyanotic heart defects. This study was performed in a tertiary cardiac center throughout the interval from May 2022 to April 2023 in cases undergoing cardiac surgery to repair congenital heart defects. Results: There was significant statistical difference found between cyanotic and acyanotic groups regarding (post operative) TSH (p-value 0.020). There was significant statistical negative correlation between (post operative) TSH with bypass time, Cross Clamp, duration of MV, ICU stay and inotropic score and a high significant statistical positive correlation between (post operative) TSH with So2 and shows significant statistical negative correlation between post operative fT3 with duration of MV, ICU stay and inotropic score. Conclusion: Monitoring thyroid function in children with congenital heart disease is crucial, as CPB affects Thyroid function tests and other body systems, with a strong relationship with ICU stay, duration of MV, and morbidity and mortality post open heart surgeries.