A follicular carcinoma of the thyroid can be difficult to differentiate from a follicular adenoma, a benign tumor of the thyroid, on the basis of histopathological findings. In such cases, the tumor is not diagnosed as malignant until distant metastasis develops. We report a case of follicular carcinoma of the thyroid that was definitively diagnosed when an osteolytic lesion developed in the right mandibular ramus. A 66-year-old woman visited our department because of gingival swelling at the posterior part of the right mandibular molar region. Diagnostic imaging revealed a mandibular lesion that had destroyed the inside of the bone and then progressed outward. In addition, a calcified lesion was found in the left lobe of the thyroid gland, and evidence suggesting multiple bone and lung metastases was also found. A biopsy of the mandibular lesion indicated that bone metastasis from a thyroid carcinoma was most likely. However, the histopathological diagnosis of the excised thyroid specimen was follicular adenoma, thus casting doubt on the possibility of metastasis from the mandibular lesion.
A segmental resection of the mandible was performed as radical treatment for the mandibular lesion because it showed a trend toward enlargement. Histopathological examination of the resected lesion showed scant cellular atypia; however, it was definitively diagnosed as a follicular carcinoma of the thyroid on the basis of osteolytic findings in the specimen and distal metastasis to the mandible.
