What Is Bilirubin?

Bilirubin Helps Break Down Food In The Digestive System

Newborn Receiving Phototherapy
In newborns, jaundice is a common condition that usually does not need treatment. In some cases, light therapy may be used to bring bilirubin levels down.. Image © Roderick Chen / First Light / Getty Images

Bilirubin is one part of bile, which is a liquid that is secreted into the small intestine as part of the digestive process. Every day, the body breaks down and replaces red blood cells. Bilirubin is a bile salt that is produced when heme (one part of hemoglobin) in the blood is broken down. As more heme is broken down, the bilirubin level in the blood increases. The liver changes bilirubin so that it can be removed from the body by being mixed with bile and then excreted from the body through stool (giving it a brown color).

The small amount of bilirubin that leaves the body through urine is what causes urine to be yellow. People with liver diseases may have increased levels of bilirubin. 

Indirect and Direct Bilirubin

Bilirubin starts out as "unconjugated" or indirect bilirubin. Indirect bilirubin is not soluble in water and it travels through the bloodstream to the liver. A small amount of bilirubin is found in the blood. Once bilirubin makes it to the liver, it is changed chemically. This change turns the indirect bilirubin into "conjugated" (or direct) bilirubin, which is water soluble.

Conjugated bilirubin is a major part of bile. Bile is a substance that breaks down fats in the digestive process, and is often yellow or brown in color. Bile is released into the small intestine by the gallbladder after a meal, when it's needed to break down the fats that are being digested.

What Happens When There's Too Much Bilirubin

Bilirubin levels can be tested through a blood test.

There are several liver diseases that can cause the bilirubin in the body to build up and get too high. Diseases of the liver can be structural (blockage), physiologic (hepatitis or cirrhosis), or benign (Gilbert's syndrome) in nature.

Bile duct blockage. Bilirubin travels through the bile ducts to get from the liver to the small intestine.

If the ducts become physically blocked, bile builds up in the liver, and bilirubin increases in the blood. Blockages could be caused by gallstones, inflammation, or a tumor.

Liver diseases. Liver disease can affect bilirubin levels and includes hepatitis (including viral hepatitis, autoimmune hepatitis, and drug-induced hepatitis) and cirrhosis (scarring in the liver caused by inflammation). When the liver is not healthy and is not working to effectively clear waste from the blood, bilirubin levels may increase.

Gilbert's syndrome. People with Gilbert's syndrome do not have enough of the enzyme that breaks down bilirubin. The result is elevated bilirubin in the blood, which may go undetected if there are no symptoms.


Too much bilirubin building up in the body is the cause of a condition called jaundice. Jaundice is when bilirubin leaks into the body tissues and turns the whites of the eyes and the skin yellowish. Once the root cause of excess bilirubin is treated, the yellowing of the eyes and skin disappears.

In newborns, jaundice is a common condition. It is often benign, but in some cases treatment might be needed to bring the bilirubin levels down.


Roy-Chowdhury N, Roy-Chowdhury J. "Gilbert syndrome." National Organization for Rare Disorders. 2015. 20 Oct 2015.

Sticova E, Jirsa M. "New insights in bilirubin metabolism and their clinical implications." World J Gastroenterol. 2013 Oct 14;19:6398–6407. doi: 10.3748/wjg.v19.i38.6398. 20 Oct 2015.

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