Medullary thyroid carcinoma (MTC) is an uncommon thyroid cancer comprising 5% to 8% of thyroid neoplasms. In contrast to common thyroid tumors, this tumor originates from the calcitonin-producing C cells. Regional metastases to cervical lymph nodes occur early in the disease, whereas distant metastasis occurs late. Common metastatic sites include the liver, bone, brain, and adrenal medulla. We present a case of MTC metastatic to the breast. We report on this case for the following reasons: (1) metastasis to the breast is an extremely rare occurrence and could be easily confused clinically and pathologically with a primary breast neoplasm and (2) this is the first reported case of detection of breast metastasis by an MTC using FDG ((18)F-fluoro-2-deoxy-D-glucose) positron emission tomography with an accompanying histologic description.