[Synthesis and investigation of substituted iodothyronines with the aim of obtaining radiochemically stable 125I-tracers of high specific activity (author's transl)].

Various substituents were tested for their ability to replace 3'-iodine of T4 without affecting binding to T4 antibodies and without exchanging with 125I during synthesis of high specific activity T4. Measured cross-reactivities of 3'-bromo-T3 and 3'-isopropyl-T3 relative to T4 were 85.6 and 8.9%, respectively, thus suggesting a high contribution of the ionised 4'-hydroxy group to antibody binding. The 3'-bromo substituent is not exchanged with 125I during labelling by the chloramine-T method. Therefore, 3'-bromothyronines are suitable starting materials for the synthesis of 3'-bromo-5'-125iodothyronines of high specific activity; these have the advantage of a long shelf life comparable to [125I]T3, because only 125I-monolabelled thyronines are produced.