A simple and inexpensive procedure for the preparation of high specific activity 125I radioiodinated triiodothyronine and thyroxine is described. The specific activities achieved are approximately 350 Ci/g for 125I-T4, and either 220 Ci/g or 500 Ci/g for 125I-T3 depending on the reagents employed. The high specific activity 125I-T4 and 125I-T3 are suitable for the routine radioimmunoassay of serum total T3 and T4. When properly stored they have a shelf life of at least seven weeks.
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