A low HDL value is defined as ≤1.04mmol/L (40mg/dl). HDL is a high-density lipoprotein, in which protein and lipid each account for 50%, is the smallest particles in the serum, the largest density of a group of lipoproteins, the role of anti-atherosclerosis, the physiological function of the peripheral tissues from the transport of cholesterol to the liver for metabolism, and then excreted from the body, the synthesis of sites is the liver and the small intestine. The reference range for HDL is >1.04mmol/L (40mg/dl), ≥1.55mmol/L (60mg/dl) is considered elevated, and ≤1.04mmol/L (40mg/dl) is considered reduced. Decreased HDL is seen for the following reasons: obesity, high sugar diet, smoking, taking certain medications (e.g., beta blockers, thiazide diuretics, androgens, etc.), and the presence of certain diseases (e.g., metabolic syndrome, diabetes mellitus). When a low HDL value occurs, it is important to actively search for the cause of the disease, early detection and early treatment.