A 63-year-old woman with hypothyroid disease was undergoing treatment with thyroid hormone. She had a bone scan for generalized bone pain and weight loss. The scan revealed a lytic lesion in the skull. Later a Tc-99m sestamibi scan showed thyroid uptake and infiltration into the neck structures. She underwent neck surgery with transoperative biopsy, which revealed anaplastic carcinoma, so a thyroidectomy was not performed. The surgical findings of the neck architecture were similar to those observed on the Tc-99m sestamibi scan. The skull radiograph, CT, and bone scan showed a skull lesion. CT and ultrasound of the neck revealed only atrophic changes (not diagnostic). The histopathologic findings of the skull biopsy were identical to those of the neck. The patient died 2 weeks after diagnosis.