Low high-density lipoprotein cholesterol (HDL-C) of 0.9 mmol/L can be seen in non-pathologic states such as smoking and drug use, as well as in disease states such as hepatitis and hypothyroidism, the former of which is not considered serious and the latter of which is more serious. HDL is an anti-atherosclerotic lipoprotein, which is considered an anti-atherosclerotic plasma lipoprotein, a protective factor against coronary heart disease, and a lipoprotein that transports cholesterol from peripheral tissue cells to the liver. HDL-C is low when it is below 1 mmol/L, so 0.9 mmol/L is low. The range of reference values may vary from hospital to hospital. Low HDL-C can be seen in non-pathological conditions, such as smoking, use of drugs such as estrogen or adrenocorticotropic hormone, etc. These conditions may be relieved by quitting smoking and stopping the drugs, so they are relatively not serious. However, low HDL-C can also be seen in disease states, such as alcoholic hepatitis, acute hepatitis, hypothyroidism, diabetes mellitus, etc. Such cases are more serious than the former and require timely intervention. It is recommended to visit a hospital and let the doctor make a judgment.