Urobilinogen is a colourless product of bilirubin reduction. It is formed in the intestines by bacterial action. Some urobilinogen is reabsorbed, taken up into the circulation and excreted by the kidney. Increased amounts of bilirubin are formed in hemolysis, which generates increased urobilinogen in the gut. In liver disease (such as hepatitis), the intrahepatic urobilinogen cycle is inhibited also increasing urobilinogen levels. Urobilinogen is converted to the yellow pigmented urobilin apparent in urine. The urobilinogen (also known as stercobilinogen) in the intestine is then oxidized to brown stercobilin, which gives the feces their characteristic color. In biliary obstruction, below-normal amounts of conjugated bilirubin reach the intestine for conversion to urobilinogen. With limited urobilinogen available for reabsorption and excretion, the amount of urobilin found in the urine is low.
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