BACKGROUND: This study aimed to investigate the relationship between serum triiodothyronine (T3) levels and coronary artery lesions in elderly patients with coronary heart disease combined with three low T3 syndrome.METHODS: A total of 986 cases of low T3 syndrome in elderly patients with coronary artery disease were defined as group A (low T3 group), and 1,074 cases with normal free triiodothyronine (FT3), free thyroxine (FT4), and thyroid stimulating hormone (TSH) in elderly patients of coronary heart disease were assigned to group B (control group).The levels of FT3, FT4, and TSH in two groups were determined.The level of coronary artery disease was evaluated by lesion count and Gensini score according to the results of coronary arteriography analysis.