AbstractIodine-123/iodine-131 (123I/131I)-metaiodobenzylguanidine (mIBG) scan is an established tool for the localization and treatment of neuroendocrine tumors such as paragangliomas (PGL). To minimize thyroid irradiation by the radioactive iodine in the mIBG preparation, blockade of thyroidal iodine uptake with high doses of stable iodine used to be given routinely as part of all mIBG protocols. As 123I is now more frequently utilized than 131I, concern about thyroid radiation has lessened and thyroid blockade is often considered unnecessary. However, in certain situations, the lack of thyroid blockade can significantly impact treatment decisions. This report describes 2 patients who had mediastinal masses incidentally discovered on CT scans, and on further evaluation were found to have symptoms suggesting catecholamine excess with mildly elevated plasma normetanephrine levels. 123I-mIBG scans were performed without thyroid blockade, which demonstrated accumulation of tracer in the masses that were therefore deemed positive for PGL. Both patients underwent surgical resection of the masses with their surgical pathology revealing ectopic thyroid tissue (ETT). These cases illustrate that if appropriate thyroid blockade is not performed, ETT concentrating radioiodine from mIBG can lead to falsely positive mIBG scans and unnecessary surgical procedures. We conclude that in the setting of a mass suspicious for PGL in a location potentially representing ETT, such as the mediastinum, thyroid blockade should be employed for mIBG protocols to avoid false positive scans caused by ETT.