Carcinoma showing thymus-like elements (CASTLE) is a rare malignant tumor of the thyroid or adjacent neck soft tissues, whose histogenesis is still debated. It may resemble other primary or metastatic poorly differentiated tumors histologically and the differential diagnosis is crucial for CASTLE has a better prognosis. However, CASTLE as a second primary tumor has not been reported in the literature. We report three cases of thyroid CASTLE, including a unique tumor following breast-conserving surgery for early-stage breast invasive carcinoma. There were two female and one male. All three tumors were located in the right lobe of the thyroid, and one tumor showed extension into the surrounding soft tissue. Histologically, all tumors showed expansive growth and consisted of cords, nests or sheets of epithelial cells divided into irregularly shaped lobules by fibrous connective tissue with lymphoplasmacytic infiltration. Focal squamous differentiation resembling Hassall's corpuscles were observed. All cases stained positively for CD5, CD117, high molecular weight cytokeratin, cytokeratin, P63, carcinoembryonic antigen and epithelial membrane antigen. Positive staining for Bcl-2 in two cases and chromogranin A in one case was noted. Ki-67 expression ranged from 15 to 25%. Thyroid transcription factor and CD3 were negative. There was no evidence of recurrent or metastatic disease at following surgery. These features demonstrated CASTLE may arise from branchial pouch remnants, the thyroid solid cell nests. CASTLE is a rare entity, awareness of its occurrence as a second primary tumor is important to avoid overtreatment because it is associated with a favorable prognosis.