THE THREE-HOUR CONVERSION RATIO: A TEST FOR HYPERTHYROIDISM

Abstract

ABSTRACT Serum organic radioiodine was determined at 3 and 24 h after an oral tracer dose of 131I. With the method used radioactive iodoamino acids (radio-IAA) rather than total protein-bound radioiodine was measured. The radio-IAA was expressed both as such (%/l serum) and as the conversion ratio (CR, % of total serum radioactivity). The results of these measurements and of 3 and 24 h % uptake determinations were compared in 181 euthyroid and 59 hyperthyroid patients sent for routine diagnostic assessment of thyroid function with 131I. The 3-h radio-IAA did not have any diagnostic value. The diagnostic accuracy of the 3-h CR equalled or surpassed that of the other tests. It was found to discriminate eu- from hyperthyroidism even in the presence of iodine deficiency (where % uptake tests fail) and small thyroidal iodine pools (where other serum tests fail). Only in renal insufficiency and in cardiac failure may the 24-h radio-IAA effect a better differentiation between hyperthyroid and euthyroid patients. A fair degree of correlation between the 3-h CR and the 24-h radio-IAA was found both in hyperthyroid and in euthyroid patients with small thyroidal iodine pools, but the variation of the 3-h CR with the 24-h radio-IAA was much smaller in the latter group. Values for several parameters tended to decrease with increasing age. Factors influencing the radio-IAA and CR are discussed. The new test should increase the diagnostic accuracy obtainable with the short-lived isotope 132I.