A measurable proxy used in a clinical trial instead of the actual health outcome you care about — useful for speed, but not always a reliable stand-in.Examples: using IGF-1 levels as a surrogate for GH axis activity; using HbA1c as a surrogate for diabetes complications; using tumor size reduction as a surrogate for overall survival. Surrogate endpoints can accelerate drug development, but a drug that improves the surrogate does not necessarily improve the clinical outcome if the surrogate is not causally linked to that outcome. The FDA has approved drugs based on surrogate endpoints that later failed to improve survival (accelerated approval pathway), which is why surrogate endpoint trials are viewed as preliminary evidence pending confirmatory outcomes data.