The present invention concerns the use of a radioiodinated phenolic compound of the formula
wherein: m and n are independently 0, 1, 2 or 3, X is a group that is negatively or positively charged at physiological pH, R, R1, R2 and R3 are independently hydrogen, C1-C4 alkyl, or a carboxyl group, and I* is 123I, 131I or 125I, and its pharmaceutically-acceptable salts. The compound is formulated and used in vivo in an animal in brachytherapy in an implantable catheter. In addition, due to the rapid renal clearance of these compounds, they may be used to study renal function. A process to prepare these compounds is also disclosed.