Bilirubin is a yellow-orange pigment your body produces when it breaks down old red blood cells. It’s the substance responsible for the yellow color of bruises, the brown color of stool, and the yellowing of skin and eyes known as jaundice. In healthy adults, total bilirubin levels in the blood fall between 0 and 1.0 mg/dL.
How Your Body Makes Bilirubin
Red blood cells live for about 120 days. When they reach the end of their lifespan, your spleen and liver break them down and release hemoglobin, the protein that carries oxygen. That hemoglobin is then converted into bilirubin through a series of chemical steps.
At this stage, the bilirubin is called “unconjugated” or “indirect” bilirubin. It doesn’t dissolve in water, so it hitches a ride through your bloodstream attached to a protein called albumin. Once it reaches the liver, enzymes modify it by attaching a sugar molecule, which makes it water-soluble. This processed form is called “conjugated” or “direct” bilirubin.
The liver then sends conjugated bilirubin into bile, which flows into your intestines. Gut bacteria convert it into other pigments that give stool its characteristic brown color. A small portion is filtered by the kidneys and leaves the body through urine, contributing to urine’s yellow tint. When this pathway is disrupted at any point, bilirubin builds up in the blood and eventually stains the skin and eyes yellow.
Normal Levels in Adults
A standard blood panel measures bilirubin in three ways:
- Total bilirubin: 0 to 1.0 mg/dL
- Direct (conjugated) bilirubin: 0 to 0.2 mg/dL
- Indirect (unconjugated) bilirubin: 0.2 to 0.7 mg/dL
Your doctor may ask you to fast for several hours before the test, and certain medications can affect results. If your levels come back slightly above the normal range with no other abnormalities, that’s often not cause for alarm, but your provider will want to determine the reason.
What High Bilirubin Means
Elevated bilirubin falls into three broad categories, depending on where the problem originates.
When too many red blood cells are being destroyed at once (a process called hemolysis), unconjugated bilirubin rises because the liver can’t keep up with processing it all. Conditions like sickle cell disease or certain autoimmune disorders can trigger this.
When the liver itself is damaged, both types of bilirubin can climb. Viral hepatitis, alcoholic liver disease, fatty liver disease, and cirrhosis all impair the liver’s ability to process and excrete bilirubin normally.
When bile ducts are blocked, conjugated bilirubin backs up into the bloodstream because it has nowhere to go. Gallstones are the most common culprit, but tumors in the pancreas or bile duct, scarring from previous procedures, and inflammatory conditions can also cause obstruction. Knowing which type of bilirubin is elevated helps doctors narrow down the cause quickly.
Gilbert’s Syndrome
One of the most common reasons for a mildly elevated bilirubin reading is Gilbert’s syndrome, a harmless genetic condition that affects roughly 5% to 13% of the population, with men affected more often than women. People with Gilbert’s syndrome have a sluggish version of the liver enzyme that conjugates bilirubin. Their levels typically fluctuate between 1 and 5 mg/dL and can spike during fasting, stress, illness, or dehydration. Occasional mild yellowing of the eyes may occur, but the condition requires no treatment.
Symptoms of Elevated Bilirubin
Jaundice, the visible yellowing of skin and the whites of the eyes, is the hallmark sign. But high bilirubin rarely shows up alone. Depending on the underlying cause, you may also notice dark-colored urine (tea or cola-colored), pale or clay-colored stool, itchy skin, belly pain, fatigue, confusion, fever, or unexplained weight loss. Dark urine and pale stool are particularly telling: they indicate that bilirubin is entering the bloodstream and urine instead of traveling through bile into the intestines.
Bilirubin in Newborns
Newborn jaundice is extremely common because babies are born with a high concentration of red blood cells, and their immature livers process bilirubin slowly. A slight yellow tint in the first few days of life is normal for most infants.
Treatment with phototherapy (blue-spectrum light that breaks down bilirubin through the skin) is started based on the baby’s age in hours, gestational age, and risk factors. For a healthy infant born near full term, treatment thresholds generally start around 10 to 12 mg/dL in the first day or two of life and rise as the baby gets older. Levels above 25 mg/dL are considered critical because, at very high concentrations, bilirubin can cross into the brain and cause permanent neurological damage. Population studies have found that this kind of severe injury, called kernicterus, occurs almost exclusively when levels exceed 30 mg/dL.
Why Low Bilirubin May Also Matter
Bilirubin isn’t just a waste product. It functions as an antioxidant, and a growing body of research suggests that unusually low levels carry their own risks. People with total bilirubin below roughly 0.5 mg/dL have a significantly higher incidence of heart attack, stroke, and death from cardiovascular causes. Low bilirubin is also linked to metabolic syndrome, chronic kidney disease, severe sleep apnea, and inflammatory bowel conditions. Obesity and fatty liver disease are particularly associated with suppressed bilirubin levels. Researchers are still working out whether low bilirubin directly contributes to these problems or simply reflects them, but the association is consistent enough that some scientists have proposed “hypobilirubinemia” as a distinct clinical concern.
What a Bilirubin Test Tells Your Doctor
A bilirubin test is rarely ordered in isolation. It’s typically part of a liver function panel that includes other enzymes and proteins. The pattern of results tells a story: a high indirect bilirubin with normal liver enzymes points toward red blood cell problems or Gilbert’s syndrome, while a high direct bilirubin with elevated liver enzymes suggests liver damage or a bile duct blockage. If obstruction is suspected, imaging like an ultrasound usually follows. If liver disease is the concern, additional blood work and sometimes a biopsy help clarify the picture.
For most people who encounter bilirubin on a lab report, the number falls within the normal range and requires no follow-up. A mildly elevated result in an otherwise healthy person is often Gilbert’s syndrome. Significantly elevated results, especially when paired with symptoms like jaundice or abdominal pain, warrant further investigation to identify the underlying cause.