[Investigation on the synthesis of 125I labelled triiodothyronine and thyroxine of high specific radioactivity (author's transl)].

The reaction conditions of the monoiodination of L-diiodothyronine and L-triiodothyronine by nonradioactive iodide have been investigated by separation of the reaction mixture on sephadex G-25 with sodium hydroxide 10 mmol/l as eluent and quantitative iodine estimation in the eluate. When labelling with 125I was performed under optimal conditions, a good yield of chromatographically pure L-triiodothyronine or L-thyroxine was obtained with a specific radioactivity between 2 and 3 Ci/mg. The synthesized labelled hormones were tested by radioimmunoassay. They enable a detection of less than 2 pg T3 and less than 5 pg T4.