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

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Abstract
The resluts 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 I131 is inversely related to the extrathyroidal inorganic iodine pool and the PBI131 is inversely related to the intrathyroidal iodine pool. Both these pools may be diminished in euthyroid persons and thus a high uptake of I131 may be associated with a high PBI131 and so lead to false diagnostic conclusions. In order to avoid diagnostic errors standard I131 tests shouldnever 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.