“An situations where the 24 hour iodine image differs substantially from the technetium image. With this technique it should be possible to evaluate the effects of washout with small doses of intravenous perchlorate. It may be possible to localize areas within the gland where there may be a binding deficit and to quantify the binding of iodine at these very early times after administration. CONCLUSIONS Both technetium-99m and iodine-123 can be used to advantage in assessing thyroid function and anatomy. They have similar principal gamma photons and their short physical half-lives result in low radiationdose, thus allowing administration of large quantities of activity. The very early uptakes of these radionuclides by the thyroid do not bear a consistent relationship and this requires further study. The clinical value of iodine-123 depends on its being consistently available at a reasonable cost. 5001361