a steroid alcohol found in animal fats and oils, bile, blood, brain tissue, milk, egg yolk, myelin sheaths of nerve fibers, liver, kidneys, and adrenal glands. It is a precursor of bile acids and steroid hormones, and it occurs in the most common type of gallstone, in atheroma of the arteries, in various cysts, and in carcinomatous tissue. Most of the body's cholesterol is synthesized by the liver, but some is obtained in the diet from animal-derived foods. Plant-derived foods are cholesterol-free. Cholesterol is not transported free in the blood but is bound to certain proteins to form lipoproteins. Two important fractions of the serum lipoproteins are high-density lipoproteins (HDL) and low-density lipoproteins (LDL).High levels of total serum cholesterol have been shown to be associated with a high risk for coronary artery disease and myocardial infarction. Research has drawn a distinction between HDL-C, the cholesterol carried on high-density lipoproteins and LDL-C, the cholesterol carried on low-density lipoproteins. The balance between HDL-C and LDL-C is more significant than the total concentration of cholesterol in the blood. The risk of coronary heart disease increases as LDL-C increases and HDL-C decreases.Because HDL-C promotes the removal of excess cholesterol from the cells and its excretion from the body, it is thought to be beneficial rather than harmful. In contrast, LDL-C picks up cholesterol from ingested fats and from cells that synthesize it in the body and delivers it to blood vessels and muscles where it is deposited in the cells. The concentration of cholesterol in cells within the linings of the arteries contributes to the build-up of atherosclerotic plaques. ( Structure and metabolism of cholesterol. From Dorland's, 2000. Structure and metabolism of cholesterol. From Dorland's, 2000. Structure and metabolism of cholesterol. From Dorland's, 2000. Blood Cholesterol. Laboratory testing of cholesterol in the blood is often used as a preliminary test for a disorder of blood lipids. Although the normal values for total blood cholesterol vary according to age, diet, and nationality, levels above 200 mg/dl indicate a need for further testing and efforts to reduce the cholesterol level. In general, as the total cholesterol level rises above 150 mg/dl, the risk for coronary artery disease gradually increases. Persons with cholesterol levels above 260 mg/dl may require medication to lower their LDL-C levels and reduce the risk of heart disease.Increased levels of cholesterol in the blood are found in cardiovascular disease and atherosclerosis, obstructive jaundice, hypothyroidism, nephrosis, and uncontrolled diabetes mellitus. Cholesterol exists in both a free and esterified form; the ratio of free to esterified cholesterol is significant in the diagnosis of certain diseases. For example, there is a markedly abnormal ratio of these two forms of cholesterol in hepatic biliary disease, infectious diseases, and extreme cholesterolemia.Decreased levels of cholesterol in the blood are noted when there is malabsorption of cholesterol from the intestinal tract as in pernicious anemia, hemolytic jaundice, hyperthyroidism, and terminal cancer.