Gilbert Syndrome is a common, harmless genetic condition in which a liver enzyme essential to the disposal of bilirubin (the chemical that results from the normal breakdown of hemoglobin from red blood cells) is abnormal. The condition has also been referred to as constitutional hepatic dysfunction and familial nonhemolytic jaundice. The enzyme abnormality in Gilbert syndrome results in mild elevations of bilirubin in the blood, particularly after starvation or dehydration. Gilbert syndrome is the result of a genetic mutation in the promoter region of a gene for the enzyme UGT1A (one of the enzymes called UGT glucuronosyltransferases that are important for bilirubin metabolism). The gene is located on chromosome 2. Other types of mutations in the same gene cause the Crigler-Najjar syndrome, which is a more severe and dangerous form of hyperbilirubinemia (high bilirubin in the blood).

People with two copies of the abnormal promoter region for the UGT1A gene (one inherited from each parent) have Gilbert's syndrome and elevated bilirubin levels, suggesting an autosomal recessive mode of inheritance. This means that both parents require the gene for expression of the abnormality in the offspring. Gilbert syndrome is a frequent finding in people in the United States and Europe. The condition is usually detected serendipitously (purely by accident) in the course of routine blood screening. The elevated bilirubin pigment can sometimes cause mild yellowing (jaundice) of the eyes. People with Gilbert syndrome are otherwise entirely normal with no other signs or symptoms. Their liver enzyme levels in blood serum are also entirely normal. Gilbert syndrome is most commonly diagnosed after puberty, when alterations in sex hormone levels cause the blood bilirubin levels to rise. Situations that aggravate elevated blood bilirubin levels (such as fasting, destruction of red blood cells, or illnesses) may be the initial factors that cause the patient to seek medical attention.