Bilirubin (formerly referred to as haematoidin) is the yellow breakdown product of normal heme catabolism, caused by the body’s clearance of aged red blood cells which contain hemoglobin.

Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases. It is responsible for the yellow color of bruises and the yellow discoloration in jaundice. It is also responsible for the brown color of feces, via its conversion to stercobilin, and the background straw-yellow color of urine via its breakdown product, urobilin. It has also been found in plants.

Bilirubin is created by the activity of biliverdin reductase on biliverdin, a green tetrapyrrolic bile pigment that is also a product of heme catabolism. Bilirubin, when oxidized, reverts to become biliverdin once again. This cycle, in addition to the demonstration of the potent antioxidant activity of bilirubin,[5] has led to the hypothesis that bilirubin’s main physiologic role is as a cellular antioxidant.