Cholesterol is also known as lipoproteins, which can be divided into high-density, low-density and very low-density types.
In the body, cholesterol provides the raw material for bile and hormones, carries fat-soluble vitamins and aids in immunity.
Cholesterol is familiar to many people as a key part of artery plaques, where the substance acts like a sort of patching cement.
Heinrich O. Wield and Adolf O.R. Windaus discovered cholesterol in 1928 in Germany.
In the second half of 1900s, scientists began investigating an apparent role for cholesterol in heart disease.
Today, the cardiovascular roles of cholesterol are still not fully understood.
Malnutrition is one major cause of cholesterol deficiency, but an otherwise sufficient diet very low in fat can also cause low cholesterol.
In some cases, low cholesterol can point to complications of kidney disease, such as inflammation and malnutrition.
Certain cancers, including lymphoma and leukemia, have been shown to reduce cholesterol levels.
A deficiency of cholesterol can be caused by malabsorption syndromes, such as that associated with celiac disease.
In cases of hyperthyroidism, in which the thyroid is overactive, the body may burn through cholesterol too quickly and create a deficiency.
Finally, the rare diseases of abetalipoproteinemia and hypobetalipoproteinemia can cause very low cholesterol.
High cholesterol can result from a diet with excessive levels of saturated fat, but levels are unlikely to rise far from normal ranges this way.
Liver diseases, such as cirrhosis and fatty liver, as well as heavy consumption of alcohol, a known liver toxin, can cause an excess of cholesterol.
Diabetes can also drive cholesterol high.
Kidney disease, obesity and Cushing's syndrome have been known to raise cholesterol to excessive levels.
Finally, hypothyroidism can cause high cholesterol.
Because hypothyroidism involves low thyroid levels, the body uses cholesterol too slowly and allows levels to rise above normal.