Serum bile acids are produced by the liver from cholesterol and secreted into the bile to assist in fat digestion and absorption. After performing their digestive role in the small intestine, bile acids are almost completely reabsorbed and returned to the liver via the portal circulation — a process called enterohepatic recycling.

Because the liver efficiently extracts bile acids from portal blood, elevated serum bile acids are a sensitive indicator that the liver is failing to process them properly. They tend to rise before standard liver enzymes in conditions like cholestasis of pregnancy, primary biliary cholangitis, and early hepatocellular disease — making them a useful early and sensitive marker of liver dysfunction.