Critical analysis of [131I]- and [125I]human thyroglobulin labels for radioimmunoassay use

[125I]- and [131I]thyroglobulin (Tg) tracers obtained by two different oxidation methods, chloramine-T (Chl-T) and lactoperoxidase (LP-ase), were analyzed to assess their suitability in the development of a RIA. Pairs of tracers which were prepared on a single day using these methods with a single source of 131I and 125I were compared. The following conclusions were reached. 1) Both 131I and 125I isotopes, using CHl-T or LP-ase oxidants, produce suitable tracers. 2) [131I]Tg can be used repeatedly for 2 weeks without repurification. 3) [125I]Tg, in contrast, has to be rechromatographed weekly on Sephadex G-200 to maintain assay sensitivity and adequate maximal binding. 4) Under these conditions, 2- or 9-day tracers with either isotope using Chl-T or LP-ase give similar Tg determinations in the serum. 5) The LP-ase-chromatographed 125I tracer seems to lead to higher maximal binding in the assay than the Chl-T-repurified tracer.