Cholesterol and triglycerides are fats in the blood that can cause hardening of the arteries if they are present in too high concentrations. Cholesterol can be divided into the "good" protective HDL, which you are actually allowed to have too much of, and the harmful LDL. If the cholesterol level in the blood is higher than what is recommended, it can deposit on the inside of the arteries. Later calcium will also deposit in the arteries and arteriosclerosis will develop.

Arteriosclerosis narrows the arteries and can in the end block the arteries completely with a blood clot. If that case the heart, brain or another organ will not receive the necessary oxygen and nutrients they need through the blood, which can result in a stroke.

If cholesterol content in the blood is efficiently reduced, you can prevent the arteriosclerosis from spreading. In some cases existing arteriosclerosis can even be diminished. Treatment can in effect prevent blood clots in the heart (coronary thrombosis), some types of stroke, reduce the necessity for by-pass operations and prolong life.

In the cells of the body little deviations in the genes controlling the cholesterol level in the blood can cause an elevated cholesterol level.
If a patient has more relatives who have suffered a blood clot in the heart at an early age, the doctor may recommend that the patient's siblings and children have their cholesterol levels checked.

An elevated cholesterol level can also be the result of other diseases like diabetes, a low metabolism, or it can be alcoholically induced.

The following levels of cholesterol and triglycerides are considered normal:

Many people have levels higher than the recommended limits, but this is not necessarily a serious problem, as it also depends on your overall risk of having a heart disease.

Source:

Ole Færgeman, Professor, Dr. Med, "Lægernes patientvejledning 2003", page 26.