To “stun” has been defined in several ways. The Webster dictionary provides synonyms of “to make senseless,” “to daze or stupefy,” or “to shock deeply.” The adjective form also means excellent or attractive. None of these definitions accurately represent “stunning” in relation to the treatment of thyroid cancer. Stunning entails that a diagnostic dose of radioiodine (131I) can sufficiently damage the thyroid, making follicular cells incapable of trapping subsequent therapeutic 131I (1,2). It is true that some of us were stupefied when the concept was first presented. Early reports implied that there would be no uptake of therapy. Thus, when a pretreatment diagnostic scan was compared to a posttherapy scan, the latter showed absence of uptake at one or more sites. Subsequently, the term was expanded to include the possibility that the percentage uptake of the therapeutic dose would be less than that of the prior diagnostic scintiscan, i.e., a quantitatively different finding. Lastly, the term “stunning” was expanded to include a poorer outcome after treatment than when there was no diagnostic dose prescribed or when an alternative imaging agent, such as 123I, is used in place of 131I (3,4).