HDL cholesterol measures the cholesterol carried by high‑density lipoprotein particles. These particles shuttle cholesterol out of tissues and vessel walls back to the liver (reverse cholesterol transport), help maintain endothelial function, and carry antioxidant and immune‑modulating proteins. In systems terms, HDL supports vascular integrity, metabolic flexibility, innate immunity, and delivery of cholesterol for steroid hormone production.Low values usually reflect impaired HDL production or function, commonly seen with insulin resistance, high triglycerides, type 2 diabetes, chronic kidney or liver disease, androgen excess, or chronic inflammation. The result is less cholesterol efflux, more atherogenic remnant particles, and a pro‑inflammatory vascular milieu. Men tend to have lower HDL than premenopausal women; values can be lower with aging and in the postpartum period.Being in range suggests effective reverse cholesterol transport, more stable endothelium, and a generally favorable cardiometabolic profile when considered alongside LDL cholesterol and triglycerides. For most adults, optimal tends to sit in the mid‑to‑higher end of the usual reference interval, without reaching extreme highs.High values usually reflect increased HDL cholesterol content or particle number from genetic variants (e.g., CETP or SR‑B1 pathways), estrogen exposure, or certain liver conditions. Very high levels can signal “dysfunctional” HDL that is less anti‑inflammatory, and epidemiologic data show a U‑shaped relationship with risk. Women and pregnancy often show higher HDL without harm when elevations are moderate.Notes: HDL‑C is a concentration of cholesterol, not a direct measure of HDL function; efflux capacity and particle composition can differ. Fasting is not required. Acute illness and inflammation can lower HDL. Estrogens tend to raise HDL; androgens, some beta‑blockers, and anabolic steroids can lower it. Assay methods and population norms vary by age and sex.
