Hypocholesterolemia

Hypocholesterolemia is the presence of abnormally low (hypo-) levels of cholesterol in the blood (-emia). While hypercholesterolemia, the presence of high cholesterol, has been linked strongly with cardiovascular disease, it is much less certain whether low cholesterol levels are intrinsically harmful.

Classification
According to most authorities, only total cholesterol levels below 160 mg/dL or 4.1 mmol/l are to be classified as "hypocholesterolemia".

Causes
Possible causes of low cholesterol are:
 * hyperthyroidism, or an overactive thyroid gland
 * liver disease
 * malabsorption (inadequate absorption of nutrients from the intestines)
 * malnutrition
 * celiac disease
 * abetalipoproteinemia - a rare genetic disease that causes cholesterol readings below 50mg/dl. It is found mostly in Jewish populations.
 * hypobetalipoproteinemia - a genetic disease that causes cholesterol readings below 50mg/dl
 * manganese deficiency
 * Smith-Lemli-Opitz syndrome

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, cancer, hemorrhagic stroke 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 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
Low cholesterol levels are predictive of clinical deterioration, and are correlated with altered cytokine levels.