The intrathyroidal thymic carcinoma is a rare neoplasm. The probable origin of this neoplasm is the presence of ectopic thymic tissue or remnants of the third and fourth branchial arch. The case of a 49-year-old female with an initial diagnosis of medullary thyroid cancer is presented. When she was operated for regional recurrence, 16 years later, the pathology report demonstrates the presence of a intrathyroidal thymic carcinoma. Intrathyroidal thymic carcinoma is considered an independent type of thyroid carcinoma because this entity has specific clinical-pathological characteristics similar to thymic carcinomas and different prognosis than known thyroid carcinomas. We present the case of a patient initially treated as having a medullary thyroid carcinoma who, upon presenting recurrence, the presence of intrathyroidal thymic carcinoma was demonstrated.