Objective To observe the changes of thyroid hormone level in patients with coronary heart disease complicated with atrial fibrillation and the related mechanism.Methods Of 150 patients with coronary heart disease,80 patients were with coronary heart disease(CAHD group)and 70 patients with coronary heart disease complicated with atrial fibrillation(CAHD+AF group).50 healthy volunteers in the same period were selected as control group.On admission,the test or assessment of blood biochemistry,carotid atherosclerosis and thyroid hormone were performed in two groups.Results The levels of thyroid hormones,blood lipids,coagulation function and the atherosclerosis thickness of CAHD+AF group and CAHD group were significantly increased when compared with the control group.The differences between CAHD+AF group and CAHD group of the level of FT4,TSH,blood lipid,blood coagulation function and degree of atherosclerosis were significant(P<0.05),.The differences between CAHD+AF group and CAHD group of the levels of TT3,TT4 and FT3had no statistical significance(P>0.05).Conclusion The decrease of FT4 and TSH levels can induce the increase of serum lipid levels,blood coagulation function and atherosclerosis,which can induce the high risk of coronary heart disease with atrial fibrillation.