123 I EARLY RAOlOlOOINE TEST AS A PREDICTOR OF OUT - COME IN CHILORENWITH THYROTOXICOSIS.

Abstract

Prediction of relapse in thyrotoxic patients have been difficult with currently available tests especially without discontinuing thlonamide therapy. We have devised a new method of analysis, using early (20 minute) radio-iodine kinetics to determine if normal pituitary feedback mechanism has returned in medically treated patients without discontinuing therapy. By using the short half life radio-isotope 123 I and thionamlde 1 hour prior to the test, we have been able to limit radiation exposure to less than 1% of that generally experienced with current use of 125 1 or 131 1. Our method uses coincidence counting of gamma and x-rays emitted by the isotope which permits determination of the absolute rate of disintegration of the isotope independent of geometry. In 20 patients tested thus far we have been able to predict the outcome correctly in 87.5% with a minimum follow up of 6 months. A trapping rate constant K₁ and also absolute iodine uptake AIU is calculated. The upper limit of normals for K₁ is 0.03 min-1 and for AIU 0.04 ug/min. In the hyperthyrold patients without remission all values before and after tri-iodothyronine suppression were well outside this normal range for both measurements. Two patients with abnormal measurements who did go into remission appear to have thyroiditis in addition to Graves' disease. This new method of analysis appears to enhance substantially the accuracy of prediction of remission in medically treated patients with Graves' disease.

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