THE DISTURBANCE IN IODINE METABOLISM PRODUCED BY THYROID SURGERY

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Abstract
IT IS generally held that the concentration of protein-bound radioiodine (PBI131) i. a sample of plasma taken forty-eight or seventy-two hours following administration of a tracer dose of I131 is a reliable index of thyroid function (1–6). It has been occasionally noted, however, that in patients successfully treated for hyperthyroidism the PBI131 tends to remain above the generally accepted level of normality and thus to indicate the presence of thyrotoxicosis in subjects who are euthyroid or even myxedematous in all clinical respects (7–9). During the past year several papers (10–13), including one from this laboratory (14), have appeared reporting more extensive results of routine radioiodine tests, and confirming that the PBI131 test and the “conversion ratio” may be frequently misleading in patients previously treated for thyroid disease by either subtotal thyroidectomy or irradiation with radioiodine. Parallel to the increase in the rate of appearance of PBI131 in the circulation, an increased rate of release of I131 by the thyroid could be observed in these patients, even if they were treated for cardiac disease and had never been thyrotoxic (14, 15). The obvious explanation was to relate the apparent increase in the turnover of I131 in the thyroid to the considerable reduction in the size of the thyroidal organic iodine pool brought about by these two forms of therapy—the I131 then being distributed in a smaller thyroidal iodine pool and a normal hormonal secretion rate corresponding to the secretion of a larger fraction of thyroidal I131.