SummaryThe authors have followed the evolution of labeled iodide and hormonal iodine in the plasma of 58 patients (including 15 normal subjects) given a test dose of I131, with the aim of assessing the diagnostic value of this test. The results were compared with those of other thyroid tests : iodine uptake by the thyroid gland, B. M. R. measurement of plasma cholesterol, etc...Hormonal iodine shows characteristic patterns, which make it possible to distinguish between hyperthyroid, normal and hypothyroïd cases, especially if the maximal values attained are taken into account. The hormone can be localised amongst protein fractions by electrophoresis of the serum of subjects injected with radioiodine.The curves of labeled iodide furnish interesting information concerning the metabolism of iodine and differ significantly in hyperthyroïd and euthyroid subjects. An attempt has been made to interpret these curves.Various applications of these test are shown and illustrated by a few examples.