CASTLE (carcinoma showing thymus-like element) is an extremely rare form of thyroid cancer. It is an extremely rare form of thyroid cancer with a very low incidence, with no more than 100 cases reported in the literature. It is believed to originate from follicular epithelial cells, embryonic cells (stem cells) and residual thyroglossal ducts, and due to changes in the local microenvironment, the original tissues are transformed and further carcinogenesis occurs. The fourth edition of WHO Oncology Classification has included CASTLE as an independent tumor of the thyroid gland into the classification. Clinically, the first symptom is a neck lump, which is usually large and located in the lower pole of the thyroid gland. The lump has no envelope and locally invades the surrounding tissues, which may cause hoarseness and difficulty in breathing and swallowing in severe cases. Pathologically, the tumor appears as a nest-like or cross-anastomosing beam-like structure of varying size, which is very similar to thymoma or thymic carcinoma. Immunohistochemistry is positive for CD5 and CD117. CASTLE is a very rare form of thyroid cancer, and the diagnosis should first exclude infiltration of tumors from adjacent organs and tissues and metastases from other sites. It also needs to be differentiated from primary squamous cell carcinoma of the thyroid, undifferentiated carcinoma of the thyroid, and medullary carcinoma of the thyroid. Treatment is mainly surgical, and total thyroidectomy is preferred. Patients with recurrent tumor can be treated by surgery again, which can still prolong the survival of patients. For patients with infiltrative growth, lymph node metastasis and postoperative recurrence, radiotherapy can be used as a supplement. The effect of chemotherapy, neoadjuvant chemotherapy, TSH suppression therapy and iodine 131 therapy is unclear or even ineffective. Post-operative follow-up is recommended for neck ultrasound for local recurrence and lymph node metastasis, chest CT for lung metastasis, abdominal ultrasound for liver metastasis, and whole-body bone scan for bone metastasis. Blood TG test and iodine 131 whole-body scan are not required as routine follow-up examinations. Overall, patients with no local invasion of peripheral tissues, no lymph node metastasis, and no distant metastasis have a relatively good prognosis.