Well-differentiated thyroid carcinoma rarely spreads to soft tissues. Thyroid carcinoma arising within a mature cystic teratoma is even rarer. We report an extremely rare case of synchronous follicular thyroid carcinoma arising within a mature cystic ovarian teratoma and stage IV differentiated thyroid carcinoma. A 62-year-old woman who lived in an iodine-deficient area was accidentally diagnosed with an ovarian cyst during a radiological metastatic work-up for thyroid cancer. Following laparoscopic left salpingo-oophorectomy, histopathological examination revealed a follicular thyroid carcinoma arising within a mature cystic teratoma. After that, total thyroidectomy and surgical resection of the soft tissue lesion in the supraclavicular fossa were performed, and the patient received subsequent 131I ablation therapy, but the disease progression was recorded 3 months later. We believe that iodine deficiency plays a role in the malignant transformation of thyroid tissues within a mature cystic teratoma. In elderly individuals with significant metastases, radioactive iodine therapy is ineffective.