Radioiodine tracer tests in the diagnosis of hyperthyroidism.

Abstract

The use of radioiodine, ,31I, has now an established place as an aid in the diagnosis of hyperthyroidism, but there is no agreement on which radioiodine test is best for this purpose. Many indices of thyroid function have been proposed which variously involve the measure ment, either serial or at fixed times after the administra tion of the radioisotope, of its accumulation by the thyroid, excretion by the kidneys, and concentration in whole blood, blood plasma, and saliva. Indices dependent on measurements of the radioiodine content in the thyroid or in blood are considered preferable to those dependent on urinary excretion because of the problems involved, particularly for out-patients, in urine collection over set periods of time. Almost all of the radioiodine tests advanced in the literature will distinguish between the frankly thyrotoxic patient and the normal subject, but the real usefulness of a test must lie in its ability to aid the diagnosis in clinically borderline cases, and in its simplicity. It is often difficult to assess the relative merits of parti cular proposed indices of thyroid function, as the clini cal material varies so widely from one group of workers to another. However, most of the more promising radioiodine indices have been compared with each other, the B.M.R., and the biochemical determination of protein-bound iodine (P.B.I.) by the Royal Melbourne Hospital group (Clarke and Aujard, 1954 ; Madigan and King, 1954 ; Winikoff, 1954 ; Clarke, Sherriff, and Wini koff, 1955) and by others (Goodwin, Macgregor, Miller, and Wayne, 1951 ; Fryers, 1956 ; McConahey, Owen, and Keating, 1956). From the studies in Melbourne it has been concluded that the best single index of thyroid function is the bio chemical determination of P.B.I., which separates both the hyperthyroid and hypothyroid groups from the nor mal or euthyroid group with a high degree of accuracy. Between 90% and 95% of the patients in each group, including a large number of clinically borderline cases, have been correctly classified by this index. The determination of P.B.I., however, is technically difficult on account of the very low levels of iodine to be measured, and demands the utmost care in keeping the laboratory and apparatus completely free from iodine contamination. For this reason it has been hoped that the use of radioiodine, which is readily detected by its beta and gamma emanations, would provide a simpler index of thyroid activity. The best radioiodine indices in the Royal Melbourne Hospital studies referred to above have been shown to be