Objective To investigate the correlation between low triiodothyronine syndrome (LT3S) and degree of coronary artery lesions in the patients with acute coronary syndrome (ACS). Methods A retrospective study was made on clinical data of 204 ACS patients in Eastern Theater General Hospital of Chinese People′s Liberation Army from August 2018 to December 2019. After excluding those with hyperthyroidism, subclinical hyperthyroidism, hypothyroidism, and subclinical hypothyroidism based on thyroid function, the remaining 189 patients were divided into normal thyroid function group (control group, n=153) and LT3S group (n=36). SPSS 25.0 was used for data analysis. Depending on the data type, t-test, rank sum test or χ² test was used for comparison between groups. Spearman correlation analysis was used to determine the correlation between free triiodothyronine (FT3) and Gensini score, number of stents and B-type natriuretic peptide. Unconditional logistic regression analysis was employed to analyze the correlation between FT3 and severe coronary artery lesions, and the receiver operating characteristic (ROC) curve was drawn to determine the value of FT3 in predicting the degree of coronary lesions. Results Gensini score, number of stents and proportion of main lesions≥50% in the LT3S group were significantly higher than those in control group (P<0.05 for all). FT3 and a history of diabetes mellitus were independent risk factors for severe coronary stenosis in ACS patients (P<0.01). Area under the ROC curves of FT3 in predicting severe coronary stenosis was 0.697, with a best truncation value of 4.195 pmol/L, a sensitivity of 69.8%, and a specificity of 70.0%. Conclusion LT3S is closely associated with the degree of coronary artery lesions in ACS patients, and FT3 can be used as an independent index to predict the degree of coronary artery lesions in them.