Hypocholesterolemia is the presence of abnormally low (hypo-) levels of cholesterol

Cholesterol

Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

 in the blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 (-emia). Although the presence of high cholesterol (hyper-cholesterolemia

Hypercholesterolemia

Hypercholesterolemia is the presence of high levels of cholesterol in the blood. It is not a disease but a metabolic derangement that can be caused by many diseases, notably cardiovascular disease...

) has been linked strongly with cardiovascular disease

Cardiovascular disease

Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...

, a defect in the body's production of cholesterol can lead to adverse consequences as well. Cholesterol is an essential component of mammalian cell membranes and is required to establish proper membrane permeability and fluidity. It is not clear if a lower than average cholesterol level is directly harmful; it is often encountered in particular illnesses.

Classification

According to the American Heart Association in 1994, only total cholesterol levels below 160 mg/dL or 4.1 mmol/l are to be classified as "hypocholesterolemia". However, this is not agreed on universally and some put the level lower.

Causes

Possible causes of low cholesterol are:

• statins
• hyperthyroidism
  Hyperthyroidism
  Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...
  
  , or an overactive thyroid gland
• adrenal insufficiency
  Adrenal insufficiency
  Adrenal insufficiency is a condition in which the adrenal glands, located above the kidneys, do not produce adequate amounts of steroid hormones , primarily cortisol, but may also include impaired aldosterone production which regulates sodium, potassium and water retention...
  
  
• liver disease
  Liver disease
  Liver disease is a broad term describing any single number of diseases affecting the liver.-Diseases:* Hepatitis, inflammation of the liver, caused mainly by various viruses but also by some poisons , autoimmunity or hereditary conditions...
  
  
• malabsorption
  Malabsorption
  Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality...
  
   (inadequate absorption of nutrients from the intestines), such as in celiac disease
• malnutrition
  Malnutrition
  Malnutrition is the condition that results from taking an unbalanced diet in which certain nutrients are lacking, in excess , or in the wrong proportions....
  
  
• abetalipoproteinemia
  Abetalipoproteinemia
  Abetalipoproteinemia, or Bassen-Kornzweig syndrome, is a rare autosomal recessive disorder that interferes with the normal absorption of fat and fat-soluble vitamins from food. It is caused by a deficiency of apolipoprotein B-48 and B-100, which are used in the synthesis and exportation of...
  
   - a rare genetic disease that causes cholesterol readings below 50 mg/dl. It is found mostly in Jewish populations.
• hypobetalipoproteinemia
  Hypobetalipoproteinemia
  Hypobetalipoproteinemia is a disorder consisting of low levels of LDL cholesterol or apolipoprotein B, below the 5th percentile. The patient can have hypobetalipoproteinemia and simultaneously have high levels of HDL cholesterol....
  
   - a genetic disease that causes cholesterol readings below 50 mg/dl
• manganese deficiency
  Manganese deficiency (medicine)
  Manganese deficiency in humans results in a number of medical problems. Manganese is a vital element of nutrition in very small quantities...
  
  
• Smith-Lemli-Opitz syndrome
  Smith-Lemli-Opitz syndrome
  Smith-Lemli-Opitz syndrome is an autosomal recessive metabolic and developmental congenital disorder that causes the inability to correctly produce or synthesize cholesterol due to a low occurrence of the 7-DHC reductace enzyme.-Symptoms:The signs and symptoms of SLOS syndrome vary widely...
  
  
• Marfan syndrome
  Marfan syndrome
  Marfan syndrome is a genetic disorder of the connective tissue. People with Marfan's tend to be unusually tall, with long limbs and long, thin fingers....
  
  
• leukemia
  Leukemia
  Leukemia or leukaemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of immature white blood cells called "blasts". Leukemia is a broad term covering a spectrum of diseases...
  
  s and other hematological diseases

Role in disease

With the increased use of medication to suppress cholesterol, some have expressed concern that lowering cholesterol levels excessively will itself cause disease.

Specific disease entities

Demographic studies suggest that cholesterol levels form an U-shape curve when plotted against mortality; this suggests that low cholesterol is associated with increased mortality, mainly due to depression

Clinical depression

Major depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and by loss of interest or pleasure in normally enjoyable activities...

, cancer

Cancer

Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

, hemorrhagic stroke, aortic dissection

Aortic dissection

Aortic dissection occurs when a tear in the inner wall of the aorta causes blood to flow between the layers of the wall of the aorta and force the layers apart. The dissection typically extends anterograde, but can extend retrograde from the site of the intimal tear. Aortic dissection is a medical...

 and respiratory diseases. It is possible that whatever causes the low cholesterol level also causes mortality, and that the low cholesterol is simply a marker of poor health.

Links with depression have been supported by studies. In contrast, no evidence was found for a link with hemorrhagic stroke (although higher cholesterol levels conferred a relative protection), and neither did statin drugs worsen the risk.

The Heart Protection Study

Heart Protection Study

The Heart Protection Study was a large randomized controlled trialrun by the Clinical Trial Service Unit, and fundedby the Medical Research Council and the British Heart Foundation in the United Kingdom...

 found no increase in either respiratory disease or neuropsychiatric illness in a large trial population taking a statin drug.

Elderly

In the elderly, low cholesterol may confer a health risk that may not be offset by the beneficial effects of cholesterol lowering. Similarly, for elderly patients admitted to hospital, low cholesterol may predict short-term mortality.

Critical illness

In the setting of critical illness, low cholesterol levels are predictive of clinical deterioration, and are correlated with altered cytokine

Cytokine

Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

levels.