Recombinant human thyroid-stimulating hormone-aided scintigraphy: comparison of imaging at multiple times after I-131 administration

PURPOSE: Recombinant human thyroid-stimulating hormone (rhTSH) may be used in lieu of thyroid hormone withdrawal for the evaluation of thyroid cancer. Scintigraphy using the existing rhTSH protocol is performed 48 hours after I-131 administration. The authors investigated the feasibility of whole-body imaging at 72 hours and evaluated thyroid tissue uptake at 48 to 144 hours. METHODS: Thirty-two patients who previously had thyroidectomy for thyroid cancer were examined. Whole-body imaging was performed routinely at 48 and 72 hours after I-131 administration. Thyroid tissue was visualized in 12 patients, and large foci were imaged for as long as 144 hours. Activity ratios for thyroid tissue to background were determined for 10 patients. RESULTS: Whole-body images at 48 and 72 hours were comparable, generally with lower background activity at 72 hours, and thyroid tissue was well visualized at all times after 48 hours. Thyroid tissue-to-background activity ratios at 72 to 144 hours were equal to or greater than those at 48 hours in 9 of 10 patients. CONCLUSIONS: Recombinant human TSH-aided whole-body scintigraphy is possible 72 hours after I-131 administration, adding flexibility and convenience to the existing protocol and permitting confirmation of findings at 48 hours. Tracer uptake in thyroid tissue persists at later times. Therefore, rapid clearance of background activity appears to be the primary cause of the previously reported decrease in radioiodine retention in euthyroid persons receiving rhTSH.