STUDIES OF STABLE IODINE METABOLISM AS A GUIDE TO THE INTERPRETATION OF RADIOIODINE TESTS

Abstract

ABSTRACT The results of radioiodine tests are dependent not only on thyroid function but also on the size of the iodine pools in which the radioiodine is diluted. The significance of the second factor is analysed in this paper and it is shown that there is a danger in interpreting radioiodine tests in isolation. The uptake of 131I is inversely related to the extrathyroidal inorganic iodine pool and the PB131I is inversely related to the intrathyroidal iodine pool. Both these pools may be diminished in euthyroid persons and thus a high uptake of 131I may be associated with a high PB131I and so lead to false diagnostic conclusions. In order to avoid diagnostic errors standard 131I tests should never be reported without some knowledge of relevant clinical features, for example, whether there is the possibility of prolonged iodine deficiency, of previous treatment, of Hashimoto's disease, or of dyshormonogenesis. In this way due weight can be given to factors which increase or decrease the iodine pools of the body and discrepancies between the clinical picture and radioiodine tests become obvious. More specific investigations can then be undertaken in appropriate cases.