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.ilirubin (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.