Objective To analyze the severity of coronary artery lesions and the prognosis in patients with low levels of free triiodothyronine (fT3)undergoing coronary angiography (CAG). Methods From August 2011 to August 2012, 600 consecutive suspected patients with coronary heart disease (CHD) who undergone CAG were selected. The patients were divided into two groups based on thyroid function prior to CAG:euthyroidism group (450 patients) and low T3 group (150 patients). All the patients underwent elective coronary angiography, then the severity of coronary artery lesions was assessed by the Gensini scores , and the patients were followed up for cardiovascular events. Results The incidence rate of CHD in low T3 group was significantly higher than that in euthyroidism group (P=0.01). Multiple variables logistic regression analysis showed that fT3 was a protective factor for CHD (OR = 0.351,P < 0.001); The severity of coronary artery lesions was greater in low T3 group than in euthyroidism group (P < 0.001). The mean duration of follow-up was (15.4 ± 3.6) months; the incidence of the composite end-point (death, non-fatal myocardial infarction and target vessel revascularization ) was significantly higher in low T3 group than in euthyroidism group (P=0.023). Conclusions Patients with a decreased T3 level has a higher incidence rate of CHD , an increase in the severity of coronary artery lesions , and a poorer prognosis.