We have been performing a systematic screening using a whole-body counter of internal radiation exposure in the affected areas by the Fukushima Daiichi nuclear incident. This case suggests 2 important points regarding the thyroid internal exposure screening immediately after a nuclear disaster. First, clinicians should recognize the possibility of overestimation of I exposure during the initial checkups of thyroid internal exposure if a detailed history of treatment by radioisotopes is not obtained. Second, in vivo measurements should be taken immediately after the disaster because all of the radioactive iodine generated after the disaster disappears within a few months.