Interference of iodine-125 ligands in radioimmunoassay: evidence implicating thyroxine-binding globulin

Although there is abundant published evidence that radioiodinated antigens interfere in digoxin radioimmunoassays, other radioimmunoassays are similarly affected. We investigated the relationship of radioiodinated antigen structure to its function in the immunoassay. Carrier-free 125I-labeled iodotyrosine and iodohistamine derivatives were incubated with human serum, and the bound and free fractions were separated. We demonstrated that diiodotyrosyl analogs bind avidly to serum proteins. Because protein binding could be reduced with competitors that inhibit thyroxine-binding globulin, such as 1,8-anilinonaphthalene sulfonate and thyroxine, thyroxine-binding globulin was clearly implicated. Diiodotyrosyl compounds also bound to solutions of purified thyroxine-binding globulin, and this binding was inhibited by the same two competitors. We postulate that thyroxine-binding globulin is the major source of the heretofore unexplained interference of radioiodinated haptens. We present recommendations for eliminating or minimizing such interference.