RESUMEN
Although medullary thyroid carcinoma (MTC) may coexist with papillary thyroid carcinoma (PTC) as a collision tumor within the same nodule or as two or more spatially separated tumors, these two carcinomas rarely coexist. We encountered three cases of sporadic MTCs spatially separated from PTCs, which occurred concurrently, either within the same thyroid lobe or in different thyroid lobes. In each of the cases the patients underwent total thyroidectomy and neck dissection. PTC metastases of the lymph node were observed in two of the cases and MTC metastasis of the lymph node was observed in one case. Among the multiple thyroid nodules affected by both MTCs and PTCs, only the dominant nodules had spread to the lymph nodes. Because MTC has a different clinical significance from PTC, in patients with multiple thyroid nodules, appropriate diagnostic approaches, such as fine needle aspiration of all possible nodules and measurement of serum calcitonin level, should be performed.