Lipids (fats), together with proteins and carbohydrates, are the main components of living cells. Cholesterol and triglycerides are lipids that are stored in the body and serve as a source of energy in addition to their role in cell structure. Desirable levels of blood fats are:
- Total cholesterol
Below 200 mg/dL
- HDL cholesterol
Men: above 40 mg/dL
Women: above 50 mg/dL
- LDL cholesterol
Below 100 mg/dL
Below 70 mg/dL for people with diabetes or heart disease
Below 150 mg/dL
When lipid levels in the bloodstream are too high or low, this condition is called dyslipidemia. The most common types of dyslipidemia are:
- High levels of low-density lipoprotein (LDL or “bad”) cholesterol
- Low levels of high-density lipoprotein (HDL or “good”) cholesterol
- High levels of triglycerides
When LDL cholesterol levels are high, fatty deposits (called plaques) can build up in the arteries, the blood vessels that carry blood from the heart throughout the body. Over time, plaques narrow the arteries, producing atherosclerosis (hardening of the arteries). This can cause heart disease, heart attack, peripheral artery disease (reduced blood flow in the limbs, usually the legs), or stroke. Low levels of HDL and high levels of triglycerides can also increase fat build-up in the arteries. High levels of HDL cholesterol, however, protect the heart by helping to remove the build-up of LDL from the arteries.
Endocrine causes of dyslipidemia
The most common lipid disorder is hyperlipidemia, high levels of blood fat. While many of its possible causes are nonendocrine, hyperlipidemia can also be related to a hormonal disease such as diabetes, hypothyroidism (low levels of thyroid hormone), polycystic ovary syndrome (PCOS), metabolic syndrome, and Cushing syndrome. Central obesity (excess fat around the waist) or insulin resistance (a condition in which the body doesn’t use insulin properly), or both—each of which are risk factors for dyslipidemia—are commonly found in patients with these endocrine disorders.
Diabetes is a disease in which blood glucose (sugar) levels are higher than normal. Insulin, a hormone produced by the pancreas, helps glucose move from the bloodstream into cells where it is used for energy. Diabetes occurs when the pancreas doesn’t produce enough insulin or when the body becomes resistant to the effects of insulin, or a combination of both. The result is that glucose doesn’t enter the cells and builds up in the blood. Over time, too much glucose in the blood can lead to increased plaque deposits on the insides of the blood vessel walls.
Because people with diabetes are at high risk for cardiovascular disease, keeping blood sugar levels close to normal is key to preventing this and other serious complications. Regulating blood pressure and lipid levels is especially important to manage cardiovascular disease risk. The most typical lipid pattern in diabetes consists of elevated triglyceride levels, low HDL levels, and small, dense LDL particles, which easily stick to artery walls. This lipid pattern is associated with central obesity and insulin resistance.
Thyroid hormone regulates how the body uses and stores energy (a process called metabolism) and plays a role in regulating lipids. Hypothyroidism (low thyroid hormone production) is the most common thyroid disorder. Its symptoms include tiredness, mental slowing, sluggishness, feeling cold, weight gain (only 5-10 pounds), dry skin and brittle hair, constipation, and menstrual irregularities. These symptoms are not unique to hypothyroidism, and many people with this disorder have only one or two symptoms. Luckily, a simple blood test can identify the condition early.
Early diagnosis is important because patients with untreated hypothyroidism have abnormal lipid levels that can lead to cardiovascular disease. These patients have high blood levels of both total cholesterol (i.e., LDL plus HDL) and LDL alone, and some have high levels of triglycerides. Hypothyroidism is treated with replacement thyroid hormone medication. Such treatment usually leads to a big improvement in dyslipidemia.
Polycystic ovary syndrome
PCOS consists of a group of symptoms and changes in hormone levels. The name comes from the fact that women with the condition often have many small painless cysts in the ovaries. Symptoms include irregular or absent menstrual periods, infertility, weight problems, acne, excess facial and body hair, thinning head hair, and brownish discoloration of the skin in the neck and upper chest. The exact cause of PCOS is unknown, but an imbalance of hormones is central. It has been called “ovarian androgen excess” because the ovaries produce male hormones (androgens) in increased amounts. Diagnosis is made on the basis of symptoms, measurement of hormone levels, and possibly ovarian ultrasound and measurement of blood glucose. Most women with PCOS have insulin resistance and many are overweight.
Dyslipidemia is common in women with PCOS. They often have increased levels of LDL cholesterol. Overweight women with PCOS may also have low levels of HDL cholesterol and high levels of triglycerides, which are typical of insulin resistance. These lipid disorders put women with PCOS at increased risk for cardiovascular disease later in life. PCOS is not curable, but its effects are manageable with medications and changes in diet and exercise.
The term metabolic syndrome denotes a cluster of metabolic risk factors, described below. Most people who have the metabolic syndrome feel healthy and may not have symptoms. Even so, they are at risk to develop life-threatening conditions like cardiovascular disease and diabetes in the future. The exact cause of the metabolic syndrome is unknown, but genetic factors, too much body fat (especially around the waist), and lack of exercise contribute to its development.
There are no well-accepted criteria for diagnosing the metabolic syndrome. It has been recommended that this syndrome be identified as the presence of three or more components:
- Elevated waist circumference
Men: equal to or greater than 40 inches
Women: equal to or greater than 35 inches
- Reduced HDL cholesterol
Men: less than 40 mg/dL, or currently taking medication to increase HDL
Women: less than 50 mg/dL, or currently taking medication to increase HDL
- Elevated triglycerides
Equal to or greater than 150 mg/dL, or currently taking medication to lower triglycerides
- Elevated blood pressure
Equal to or greater than 135/85 mm Hg, or currently taking medication to reduce BP
- High blood glucose
Fasting glucose equal to or greater than 100 mg/dL, or currently taking medication to lower glucose levels
The primary goal of treatment is to reduce the risk of cardiovascular disease and diabetes. This includes quitting smoking and reducing LDL cholesterol, blood pressure, and glucose to recommended levels. Lifestyle changes also help reduce the metabolic risk factors: losing weight (ideally, a body mass index less than 25), eating a healthy diet (with reduced intake of fats and cholesterol), and increasing physical activity (at least 30 minutes of moderate intensity exercise most days of the week).
Many patients with endogenous Cushing syndrome have hyperlipidemia due to their prolonged exposure to excessive cortisol. One reason for this lipid disorder is that cortisol affects where fat is stored, and when cortisol levels remain elevated, there is central fat accumulation. See the section on Cushing Syndrome and Hypertension for details about causes, diagnosis, and treatment of Cushing syndrome.
Robert M. Carey, MD
Gordon H. Williams, MD
William F. Young, Jr., MD