BACKGROUND Metastasis to the breast is rare. We herein report a patient with metastatic medullary thyroid carcinoma to the breast for whom measuring the calcitonin level was an important clue to the correct diagnosis. METHODS A 54-year-old woman visited our hospital for the treatment of recurrent metastatic medullary thyroid carcinoma due to multiple endocrine neoplasia 2A and breast cancer. Positron emission tomography performed before the operation for metastatic medullary thyroid carcinoma recurrence in the neck showed the accumulation of 18F-fluorodeoxyglucose in the bilateral breast at sites other than the disease in the neck. Ultrasonography revealed multiple tumors in both breasts. A core needle biopsy of three breast tumors was performed. Microscopically, the tumor cells showed solid growth and did not show a tubular structure. She was diagnosed with triple-negative invasive ductal carcinoma. Post-operative positron emission tomography was performed as the serum calcitonin level increased after the operation. The accumulation of 18F-fluorodeoxyglucose in the bilateral breast tumors and lymph nodes in the neck was noted. The possibility of the breast tumors being metastasis of metastatic medullary thyroid carcinoma was considered. Needle aspiration was performed for three breast tumors. The calcitonin level of the washout fluid was measured and found to be ≥ 17,500 pg/mL. Immunohistochemistry showed that the tumor cells were calcitonin-positive and gross cystic disease fluid protein-15-negative. Vandetanib was started as recurrent metastatic medullary thyroid carcinoma with breast metastasis was finally diagnosed. The serum calcitonin level decreased after 1 month. CONCLUSIONS Although breast metastasis of medullary thyroid carcinoma is rare, a correct diagnosis is indispensable for appropriate treatment. When a breast tumor shows atypical morphological features for breast cancer according to the histopathology in a patient with a history of cancer, metastasis to the breast should be considered. Calcitonin measurement of the needle washout fluid was useful for confirming metastatic medullary thyroid carcinoma.