Medullary thyroid carcinoma is a tumor of C (Parafollicular) cells and comprises approximately 5-10% of thyroid carcinomas. About 80% of cases are sporadic and the remaining 20% are inherited. Grossly, the tumor is generally solid, firm, and well-circumscribed as seen in this image. The cut surface ranges from gray to brown to yellow tan in appearance. Most cases are unencapsulated; however, rare cases with a complete fibrous capsule have been documented. Tumors less than 1.0 cm in size are called medullary microcarcinoma. The most frequent location is midportion or upper half of the gland which corresponds to the greatest concentration of C cells.