RESEARCH STARTER

Bilirubin

Bilirubin is a brownish-yellow substance found in bile, which is produced by the liver and stored in the gallbladder. This liquid plays a crucial role in digesting fats by being released into the small intestine. Bilirubin is generated through the breakdown of old or damaged red blood cells and exists in two forms: indirect (unconjugated) bilirubin, which is not water-soluble, and direct (conjugated) bilirubin, which is water-soluble and can be eliminated by the body. Elevated levels of bilirubin, known as hyperbilirubinemia, can lead to jaundice, characterized by a yellow tint in the skin and eyes. This condition is particularly common in newborns, whose immature livers may struggle to process bilirubin efficiently. Various medical issues, including liver diseases and gallbladder disorders, can lead to increased bilirubin levels, while certain medications may also affect these levels. Doctors often conduct blood tests to measure bilirubin concentrations, aiding in the diagnosis of underlying health problems.

Authored By: Campbell, Josephine 1 of 4
Published In: 2024 2 of 4
Related Topics:
Bile;Blood transfusion;Jaundice;Red blood cells (RBCs)
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Full Article

Bilirubin is a substance found in bile. Bile is a thick liquid containing bile acids that help the body digest fats. The liver produces bile, which is stored in the gallbladder. Bile is released into the small intestine to help digest food. Bilirubin is brownish-yellow in color.

An overabundance of bilirubin can cause a number of health issues and can help doctors detect other medical problems, such as liver disorders. Bilirubin levels can also help doctors judge how well the liver is working. Medical experts may request blood tests to determine the amount of bilirubin and diagnose health issues such as liver problems or some types of cancer.

Overview

Bilirubin circulates throughout the body in the blood. This is known as indirect, or unconjugated, bilirubin. Old or damaged red blood cells are broken down mainly in the spleen and liver, where bilirubin is formed. The liver processes the indirect bilirubin and makes direct, or conjugated, bilirubin. In this form, the bilirubin is attached to glucuronic acid and is soluble, meaning it can dissolve in water. Converting it to a soluble form allows the body to eliminate the excess bilirubin.

The bile containing direct bilirubin is dispensed into the digestive system through the bile ducts. In the digestive tract, bacteria transform it to urobilinogen. Some urobilinogen is reabsorbed into the blood, while the rest leaves the body in feces and is changed into pigments that give stool its normal color.

Sometimes the body has too much bilirubin, a condition called hyperbilirubinemia. This can cause jaundice, which is when bilirubin collects in the body’s tissues. The most recognizable sign of jaundice is a yellow tint to the skin and eyes.

Newborns sometimes have too much bilirubin because the red blood cells are breaking down at a high rate and their livers are not yet efficiently removing bilirubin from the blood. Too much bilirubin in infants can cause serious problems, including brain damage or kernicterus. Doctors check bilirubin levels in newborns with a skin test or a blood test twenty-four to forty-eight hours after birth, or before discharge if that happens first. Jaundice is common in newborns and often goes away quickly, but some cases may need medical evaluation. Jaundice that appears in the first twenty-four hours after birth needs prompt testing and medical evaluation. If the condition does not go away on its own, infants must be treated to reduce bilirubin levels. Infants with too much bilirubin may need light treatment or, in rare severe cases, blood transfusions. Severe cases may also need emergency care before a transfusion is considered.

A number of medical conditions may cause high levels of bilirubin. These include gallbladder disorders, infections, inherited conditions that affect the liver, hepatitis and other diseases that damage the liver, gallstones and other causes of bile duct blockage, and conditions that destroy red blood cells. Some medications, including some antibiotics and birth control pills, can also raise bilirubin levels. Low levels of bilirubin may be due to medications such as phenobarbital and theophylline.

Intestinal bacteria can change conjugated bilirubin back to unconjugated bilirubin and glucuronic acid. This, along with slowed bile flow, can help form calcium bilirubinate stones. These stones can block the bile duct, causing jaundice and pain. Doctors may order blood tests to measure indirect, direct, or combined total bilirubin levels.

Bibliography

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Kyalwazi, Beverly, and Victoria Kusztos. “Bilirubin Pathways and Pitfalls: From Processing to Pathology.” AASLD, 14 Apr. 2025, www.aasld.org/liver-fellow-network/core-series/back-basics/bilirubin-pathways-and-pitfalls-processing-pathology. Accessed 31 Mar. 2026.

Lightner, David A. Bilirubin: Jekyll and Hyde Pigment of Life. Springer, 2013.

Rodak, Bernadette F., et al., editors. Hematology: Clinical Principles and Applications. Saunders Elsevier, 2007.

“What Are Jaundice and Kernicterus?” Centers for Disease Control and Prevention, 8 Dec. 2020, archive.cdc.gov/www_cdc_gov/ncbddd/jaundice/facts.html. Accessed 31 Mar. 2026.