Aim To study the effect of low T3 syndrome on the prognosis of patients with acute myocardial infarction undergoing percutaneous coronary intervention (PCI). Methods From January 1,0 to October 1,4, 1604 patients with acute myocardial infarction diagnosed in the Department of Cardiology of Shengjing Hospital were enrolled in this study. They were divided into two groups according to whether they were complicated with low T3 syndrome:low T3 syndrome group (n=107) and normal thyroid function group (n=1497). Their general date, coronary angiographic data, PCI data, postoperative medication, and end-point events were recorded. The clinical characteristics and prognosis of the two groups were analyzed retrospectively. Results During the follow-up period, there were significant differences in all-cause mortality and cardiac mortality between the two groups (P＜0.05). COX analysis showed that low T3 syndrome was independently associated with all-cause mortality (HR＝2.7,5%CI was 1.039～4.393, P＝0.039) and cardiac death (HR＝2.6,5%CI was 1.379～5.669, P＝0.004). Conclusion Patients with acute myocardial infarction combined with low T3 syndrome undergoing PCI had worse prognosis.