Indoxyl sulfate is a dietary protein metabolite and also a metabolite of the common amino acid tryptophan. It has been identified as a uremic toxin according to the European Uremic Toxin Working Group. It is a circulating uremic toxin stimulating glomerular sclerosis and interstitial fibrosis. Indoxyl sulfate is one of the well-known substances of a group of protein-bound uremic retention solutes. Indoxyl sulfate increases the rate of progression of renal failure. In plasma, indoxyl sulfate is a protein-bound uremic solute that induces endothelial dysfunction by inhibiting endothelial proliferation and migration in vitro. Some studies suggest that indoxyl sulfate is also involved in oxidative stress. In hemodialyzed patients, serum levels of indoxyl sulfate are associated with levels of pentosidine, a marker of carbonyl and oxidative stress. In vitro, indoxyl sulfate increases reactive oxygen species (ROS) production in tubular cells and increases NAD(P)H oxidase activity in endothelial cells. Indoxyl sulfate impairs osteoblast function and induces abnormalities of bone turnover. Indoxyl sulfate strongly decreases the levels of glutathione, one of the most active antioxidant systems of the cell. Indoxyl sulfate is a microbial metabolite found in Escherichia.