Evaluation of Diagnostic Accuracy of 4-hour vs. 4- and 24-hour Radioactive Iodine Uptake Measurements in Thyrotoxic Patients

1317 Background: : 24-hour iodine-123 sodium iodide thyroid uptake is widely accepted as a standard measure of thyroid function in clinical practice. Previous joint publications by the American College of Radiology (ACR), Society of Pediatric Radiology (SPR), and Society of Nuclear Medicine (SNM) provide guidelines for thyroid uptake measurement and suggests an additional 4 to 6 hour uptake measurement to evaluate individuals with suspected rapid iodine turnover.(1,2) The objective of this retrospective study was to evaluate the correlation of diagnosis based on thyroid uptake and scan with iodine-123 at 4-6 hours alone versus combined 4- and 24- hour methodology. Methods: A single institution, retrospective study was performed to evaluate reproducible interpretative analysis of 4-6 hour iodine-123 thyroid uptake and scan to conventional 4-6 and 24-hour uptake approach in patients with clinically suspected thyroid disease. Records of 497 subjects who underwent thyroid scintigraphy for evaluation of thyrotoxicosis were evaluated from OCT 2011 to SEP 2017. 369 patients were managed based upon 4-hour thyroid uptake measurements alone in concert with unequivocal clinical exam findings and thyroid function lab data per institutional SOP. Of those patients called back for next day uptake measurement (n=128), 31 subjects were excluded for the following reasons: measurements were performed outside either the 4-6 or 22-26 hour window (n=20); pediatric population (n=1; 16 years old); previous radioactive iodine ablation (n=4); previous partial thyroidectomy (n=2); no available electronic medical records (n=2); receiving PTU therapy when thyroid function panel was performed (n=1); taking levothyroxine 1-3 days prior to thyroid uptake scan (n=1). A panel of 3 board certified Nuclear Medicine physicians individually reviewed the records (prior thyroid ultrasound imaging reports and thyroid function panels within the previous six months) of the remainder 97 eligible patients in concert with combined 4- and 24- hour thyroid uptake measurements. Patients were diagnosed as either (A.) normal/euthyroid, (B.) Graves disease, (C.) toxic nodule/nodular goiter, or (D.) thyroiditis based upon 4-hour thyroid uptake measurements. Subsequently, approximately one week later, these same patient records were re-diagnosed with 22-26 hour thyroid uptake measurements included. Patient records were anonymized and randomized in each data set to reduce the likelihood of recall. Both diagnoses for each patient were compared using Cohen’s Kappa to evaluate agreement between the two methods. Results: Diagnoses concurred between the 4-hour and 4-/24-hour uptake thyroid measurements in 76/97 patients (78.4%; Kappa=0.67). In the disparate cases, the most changed diagnosis (7/21 disagreements) was first interpreted as toxic nodular/nodular goiter and subsequently categorized as Graves disease (with nodular features). Most patients initially diagnosed with Graves disease (43/45, 96%; 95% CI 84.3%-99.5%) remained unchanged, with the exception of 2 patients later diagnosed as euthyroid (4 and 23-24 hour thyroid uptake measurements of 5.7/14.3 and 8.1/17.3, respectively). Of those initially diagnosed as euthyroid/normal (n=15), five were reclassified as either Graves (n=2) or thyroiditis (n=3). Of those initially diagnosed as thyroiditis (n=7), three were reclassified as Graves. Conclusion: 4-6 hour thyroid uptake measurements were reliable in the diagnosis of Graves disease and could potentially replace 24-hour thyroid uptake measurements in those patients. Otherwise, patients evaluated for thyrotoxicosis with equivocal lab values and imaging warrant standard 24-hour thyroid uptake measurements for proper diagnoses.