How is indirect bilirubin transported in serum?

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Indirect (unconjugated) bilirubin is hydrophobic and not water-soluble, so it cannot circulate freely in serum. It binds tightly to albumin, a serum protein, forming a protein-bound complex that safely transports bilirubin to the liver for conjugation. In the liver, it is taken up and converted to a water-soluble direct bilirubin for excretion into bile. If bilirubin were free, it could deposit in tissues; if it formed a water-soluble complex, that would imply it’s already conjugated; if it were bound to red blood cells, that would not reflect how it travels in plasma. Therefore, protein-bound transport best explains indirect bilirubin in serum.