An individual is considered to have hypolipidemia if his total cholesterol levels are below 120 mg/dL and LDL cholesterol levels (also known as your bad cholesterol) below 50 mg/dL without the use of cholesterol-lowering medications. There are many causes of low cholesterol levels, including certain heath conditions and inherited conditions.
Source:
The Merck Manual of Diagnosis of Therapy, 18th edition. 2006.

