Estimation of the Absolute Thyroid Uptake of Stable127I Using a Simplified Method and Comparison of the Results with Two Classic Methods

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Abstract
A simplified technique is described for the determination of absolute uptake of stable iodine 127I by the thyroid. The technique has been compared with 2 classic methods in 114 subjects in most of whom the radioactive uptake did not agree with the clinical findings (hyperthyroidism cured by 131I, simple goiter, patients saturated with stable iodine). The technique is based on the existence of a log-log relationship between the early radioactive iodine uptake and the thyroid clearance and on the relative constancy of renal iodine excretion rate constant. The validity of these relationships is discussed with regard to the experimental results obtained. The proposed method only necessitates the measurement of radioactive uptake at 2½ hr and the quantity of stable iodide excreted in the urine between 0 and 2½ hr. The difference between the results obtained using the simplified method and those obtained using the classic methods based on the equality of specific activities is negligible in all groups of patients studied. The diagnostic accuracy of the method is comparable to that of the classic methods. Its simplicity and rapidity allow the absolute uptake of stable iodide to be determined routinely in all patients at the same time as the classic thyroid investigations are carried out. It appears to be particularly useful in evaluating thyroid activity in those patients in whom radioactive uptake does not agree with the clinical state, especially in the follow up of hyperthyroidism treated with 131I and in simple goiters.