3',3'',5',5''-Tetraiodophenolsulfonphthalein, also known as iodophthalein, is an iodinated derivative of phenolsulfonphthalein primarily used as a radiocontrast agent in medical imaging. It enhances visibility of the gallbladder and biliary tract during X-ray examinations (cholecystography) due to its high iodine content, which effectively absorbs X-rays. This application aids in diagnosing conditions such as gallstones, obstructions, or tumors in the gastrointestinal and biliary systems. Historically administered orally or intravenously, it has been largely replaced by safer modern contrast agents. Limited use may occur in specialized laboratory settings for its iodinated properties in analytical procedures.