EVALUATION OF EARLY (5 TO 6 HOURS) IODINE-123 UPTAKE FOR DIAGNOSIS AND TREATMENT PLANNING IN GRAVES-DISEASE

Background: Twenty-four-hour radioactive iodine uptake measurements ne cessitate extra visits and time delays in diagnostic confirmation of a nd therapy planning for hyperthyroid patients. We evaluated the early (5 to 6 hours) measurement of iodine 123 uptake (EU) to predict late ( 24 hours) uptake (LU) and assessed its value in the management of hype rthyroidism. Methods: We conducted a prospective study in 51 previousl y untreated hyperthyroid and 27 euthyroid patients (initial evaluation group). Patients underwent both 6- and 24-hour I-123 uptake measureme nts. A subsequent 21 patients with Graves' disease (confirmation group ) were evaluated in light of regression data generated in the initial evaluation group. Results: An EU value of greater than 20% had a sensi tivity of 100%, a specificity of 96%, and a positive predictive value of 98% for the diagnosis of hyperthyroidism and was superior to the mo st predictive LU value (>30%), which had a sensitivity of 98%, a speci ficity of 89%, and a positive predictive value of 94%, in distinguishi ng the hyperthyroid patients from euthyroid patients or those with sub acute thyroiditis. Regression analysis revealed that the 24-hour uptak e of the hyperthyroid patients could be predicted from the early measu rement with the following formula: LU=28.94+O.584 (EU). The measured E U of the confirmation group was used to calculate a predicted LU with use of this formula. Measured LU and predicted LU correlated well (r=. 85, P<.001). Iodine 131 dose calculations were performed post hoc; LU calculated doses correlated with predicted LU doses (r=.91, P<.001). M ean dose differences were small. Conclusions: The EU of I-123 can repl ace 24-hour uptake measurements. Early uptake measurement is reliable and clinically useful for diagnosis confirmation and treatment plannin g in thyrotoxic patients.