THE DISTURBANCE IN IODINE METABOLISM PRODUCED BY THYROID SURGERY

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.