The non-thyroidal illness syndrome always occurs in the post-operative period following cardiac surgery in adults and children. Low T3 syndrome is the most common one. This abnormal thyroid hormone metabolism can significantly affect the stability of hemodynamics and the recovery of cardiac function after operation.There remains debate about the potential benefits of the treatment of the non-thyroidal illness syndrome with thyroid hormone supplementation. In this paper, the clinical manifestations and related intervention measures of low T3 syndrome in various heart operations were reviewed through previous clinical experiments.