Medullary thyroid cancer is a rare malignant neoplasm of the epithelium that derives from the C cells (parafollicular) and occurs spontaneously or as part of multiple endocrine neoplasia syndrome. The diagnosis is made by histopathology and clinical history. Might be familiar due to genetic mutation which can be identified by the genetic testing ret oncogene. The main marker is Calcitonin in blood. Prophylactic surgery in children and youth is indicated in MEN 2 (Multiple Endocrine Neoplasia) .In this study we present a case of Ca Medullary Thyroid diagnosed post operatively and where the family including sons, brothers and nephews were studied for the ret oncogene mutation. Results showed that 2 of the children were positive and due to the diagnosis were operated. One showed medullary Thyroid Ca associated hyperparathyroidism and the other was negative. Ret proto oncogene mutation studies were performed in patients with this pathology in Paraguay for the first time. The association of Thyroid Medullary carcinoma and hyperparathyroidism is rare and both were resolved with surgery and ablative therapy with 131I-MIBG