Gilbert's Syndrome: A benign condition characterized by a mild increase in unconjugated bilirubin due to a partial deficiency of the enzyme uridine diphosphate glucuronyltransferase (UGT1A1). It is the most common cause of isolated hyperbilirubinemia and typically presents with intermittent jaundice, especially during periods of stress, fasting, or illness.

Hemolysis: Conditions that lead to the breakdown of red blood cells can cause an increase in indirect (unconjugated) bilirubin. Examples include autoimmune hemolytic anemia, hereditary spherocytosis, and glucose-6-phosphate dehydrogenase (G6PD) deficiency. These conditions often have other clinical manifestations such as anemia, reticulocytosis, and sometimes spleen enlargement.
Drug-induced Hyperbilirubinemia: Certain medications can cause an increase in bilirubin levels, either by increasing bilirubin production (e.g., rifampicin) or by inhibiting its conjugation (e.g., probenecid).
Crigler-Najjar Syndrome: A rare genetic disorder affecting the UGT1A1 enzyme, leading to severe unconjugated hyperbilirubinemia. However, it's less likely than Gilbert's syndrome due to its rarity and typically more severe presentation.

Hemolytic Disease of the Newborn (HDN): In newborns, this condition can lead to severe jaundice due to maternal antibodies against the baby's red blood cells. It's critical to diagnose and treat promptly to prevent kernicterus.
Pulmonary Embolism: Though less directly related, significant pulmonary embolism can lead to hypoxia, which in turn can cause an increase in bilirubin levels. The connection is indirect, but the condition is life-threatening and should not be missed.
Sepsis: Sepsis can cause both conjugated and unconjugated hyperbilirubinemia through various mechanisms, including hemolysis, liver dysfunction, and decreased bilirubin clearance. It's a critical diagnosis to consider due to its high mortality rate if untreated.

Dubin-Johnson Syndrome: A rare genetic disorder characterized by chronic jaundice due to an inability to secrete conjugated bilirubin into the bile. It typically presents with a distinctive black liver due to the accumulation of epinephrine metabolites.
Rotor Syndrome: Similar to Dubin-Johnson syndrome but without liver discoloration. It's a benign condition with a similar pathophysiology, involving a defect in the hepatic excretion of bilirubin glucuronides.
Lucey-Driscoll Syndrome: A rare condition of transient neonatal hyperbilirubinemia, thought to be related to a maternal factor that inhibits bilirubin conjugation in the newborn.