March 27, 1995

Evaluation of Early (5 to 6 Hours) Iodine 123 Uptake for Diagnosis and Treatment Planning in Graves' Disease

Author Affiliations

From the Department of Medicine, Wright State University School of Medicine, Dayton, Ohio. Dr Hennessey is now with the Division of Endocrinology, Brown University School of Medicine, Rhode Island Hospital, Providence. Dr Berg is now with the Endocrine Clinic, Memphis, Tenn. Dr Ibrahim is now in private practice in general internal medicine in Dearborn, Mich.

Arch Intern Med. 1995;155(6):621-624. doi:10.1001/archinte.1995.00430060083010

Background:  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.

Methods:  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.

Results:  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.

Conclusions:  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)