The availability of radioactive iodine has unquestionably revolutionized the diagnosis and therapy of thyroid disease. There is less certainty, however, as to the value and meaning of I131diagnostic studies in the hyperthyroid patient following surgery or the administration of therapeutic doses of radioiodine. Goldsmith,1in 1954, stated that the thyroidal radioiodine uptake cannot be interpreted in the usual way following treatment with radioiodine; Chapman, Maloof, Maisterrena, and Martin2observed that the thyroid I131uptake was at times disproportionately low when compared to the clinical status, while others3,4noted persistently elevated thyroid uptakes or PBI131values in clinically euthyroid patients. Nodine and his collaborators5have further documented this problem with six cases in which there was a discrepancy between the clinical status and diagnostic tests with the use of I131. On the other hand, Schultz and Zieve6maintained that the thyroidal I131uptake and PBI are prognostically