Indoxyl sulfate, also known as 3-indoxylsulfate and 3-indoxylsulfuric acid, is a metabolite of dietary L-tryptophan that acts as a cardiotoxin and uremic toxin.

[1][2][3] As a uremic toxin, it stimulates glomerular sclerosis and renal interstitial fibrosis.

[3] Indoxyl is produced from indole via enzyme-mediated hydroxylation in the liver;[3][4] in vitro experiments with rat and human liver microsomes suggest that the CYP450 enzyme CYP2E1 hydroxylates indole into indoxyl.

Occasionally in urinary tract infections, bacteria produce indoxyl phosphatase which splits indoxyl sulfate forming indigo and indirubin creating dramatic purple urine.

[9] Indoxyl sulfate is also a product of indole metabolism, which is produced from tryptophan by intestinal flora, such as Escherichia coli.