Twenty-four-hour radioactive iodine uptake measurements necessitate extra visits and time delays in diagnostic confirmation of and 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 hyperthyroidism.
We conducted a prospective study in 51 previously untreated hyperthyroid and 27 euthyroid patients (initial evaluation group). Patients underwent both 6- and 24-hour123I uptake measurements. A subsequent 21 patients with Graves' disease (confirmation group) were evaluated in light of regression data generated in the initial evaluation group.
An EU value of greater than 20% had a sensitivity of 100%, a specificity of 96%, and a positive predictive value of 98% for the diagnosis of hyperthyroidism and was superior to the most predictive LU value (>30%), which had a sensitivity of 98%, a specificity of 89%, and a positive predictive value of 94%, in distinguishing the hyperthyroid patients from euthyroid patients or those with subacute thyroiditis. Regression analysis revealed that the 24-hour uptake of the hyperthyroid patients could be predicted from the early measurement with the following formula: LU=28.94+0.584 (EU). The measured EU 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). Mean dose differences were small.
The EU of123I can replace 24-hour uptake measurements. Early uptake measurement is reliable and clinically useful for diagnosis confirmation and treatment planning in thyrotoxic patients.(Arch Intern Med. 1995;155:621-624)