Intestinal permeability (IP) is part of the mucosal barrier function allowing small molecules to pass through the tight junctions between epithelial cells. In a healthy state, low IP contributes to a homeostatic immune response. In a diseased state, increased IP can lead to the permeation of luminal antigens that exacerbate intestinal immune responses. IP is tested by administrating inert IP markers orally and quantifying their percentage recoveries in urine or blood. Despite proving abnormal IP in a variety of canine intestinal disorders, IP tests have not found widespread clinical use. Currently, iohexol is regarded as a highly promising marker, as it avoids problems associated with radioactive or sugar IP markers.