The normal reference range for homocysteine in normal human serum is 5-15 μmol/L. Homocysteine levels are higher in men than in women and increase gradually as the body ages. Abnormally high homocysteine levels are considered to be caused by hyperhomocysteinemia, which is a common cause of kidney failure, congenital genetic factors, and chronic malnutrition. High homocysteine levels in human blood make the body more susceptible to endothelial cell damage, resulting in decreased elasticity of vascular tissue and inflammatory diseases, which in turn cause atherosclerosis and ischemic heart disease, and high homocysteine levels are one of the risk factors for coronary artery disease.