Urobilinogen is a colorless substance produced from the breakdown of bilirubin. In the intestines, it is converted to stercobilin by normal resident bacteria. Some is also absorbed from the intestines and goes to the kidneys for excretion, and some is converted to urobilin. Urobilin is responsible for giving urine its light yellow color, and stercobilin gives the feces its brownish appearance.
Bilirubin comes from the breakdown of hemoglobin, the oxygen carrying component of red blood cells (RBCs). RBCs often stay in the circulation for approximately 120 days, after which they die and break down. Their death precipitates the production of the unconjugated or insoluble form of bilirubin.
Unconjugated bilirubin goes to the liver, where it is converted to its conjugated or soluble form. Conjugated bilirubin becomes a component of bile, a greenish-yellow fluid produced by the liver and stored in the gallbladder. Eating foods rich in fats usually stimulates the gallbladder to release some of the bile to help in the digestion of fats, and it travels from the organ down to the small intestines.
As bile reaches the intestines, the bilirubin present in it will once again undergo conversion with the help of resident bacteria that are normally present there. This conversion will result in the formation of urobilinogen. Some is absorbed by the intestines and goes to the kidneys for excretion. A little is converted to urobilin, which is also excreted in urine. The remaining amount that was not absorbed by the intestines is usually converted to stercobilin and excreted in the feces.
An increased urobilinogen concentration in the urine is mostly indicative of hyperbilirubinemia, which is the presence of excessive amounts of bilirubin in the blood. The condition is sometimes seen in newborn babies, where bilirubin in its insoluble form accumulates in the child’s brain and causes seizures, visual problems, and mental retardation. Hemolytic anemia, which is a condition characterized by abnormal and premature death of red blood cells, also increases levels in the urine.
Other factors that may significantly increase levels of this substance are inflammation of the liver and cirrhosis. Its level in the urine becomes abnormally low when there is an obstruction in the passageway of bile. The use of broad-spectrum antibiotics that could result in the death of resident bacteria in the intestines may also lead to low urobilinogen levels.