Quantitative studies of iodine metabolism in thyroid disease

The only known function of iodine in the body is to take part in thyroid hormone synthesis, therefore iodine metabolism and thyroid function are inextricably linked. By combining radioisotopic and chemical iodine measurements a full picture of iodine metabolism can be obtained, including the plasma inorganic iodine metabolism can be obtained, including the plasma inorganic iodine concentration and the absolute uptake of stable iodine by the thyroid. Measurement of these parameters permits a much better estimate of thyroid function to be obtained with radioisotopic methods alone. The studies have provided new information about thyroid function in health, and the influence on iodine metabolism of age, sex and the level of the iodine stores of the body. Serial measurements, and extension of the studies to Icelanders, have resulted in a clearer understanding of the influence of varying levels of dietary iodine intake on the absolute iodine uptake by the thyroid, and on thyroid hormone production. A new assessment of the iodine requirements in man has been made, basing the estimate on quantitative measurements of iodine metabolism. Investigation of patients with non-toxic goiter using these methods showed characteristic patterns of iodine metabolism indicative respectively of iodine-deficiency goiter, auto-immune thyroiditis, dyshormonogenesis and iodine-induced goiter. The majority of cases of non-toxic goiter in the West of Scotland were found to have iodine deficiency. Standard radioiodine tests (thyroid uptake and PBI131) may give misleading results suggesting hyperthyroidism where it does not in fact exist. Thus a small extrathyroidal iodide pool raises the radioiodine uptake, and a small intrathyroid iodine pool raises the PBI-131. When both pools are small radioiodine tests are strongly suggestive of thyrotoxicosis, as has happened in several of our euthyroid patients. These conditions are readily detected and differentiated from thyrotoxicosis by stable iodine studies. Estimations of the thyroid radioiodine clearance, the PII and the AIU can be made using I132. In this way the patients receive very much less radiation than with the standard radioiodine tests, which require I131. This is especially important when carrying out tests on young patients, and for repeated studies on the same patient. Reduction in radiation hazard is associated with increased diagnostic accuracy, and (in conjunction with PBI estimations) these stable iodine studies provide the best laboratory aid at present available for the investigation of problem cases of thyroid dysfunction