Carcinoma medullary thyroid is a rare, malignant neoplasm. Affected patients can be hereditary carriers, but in most cases they are spontaneously affected. In hereditary cases, prophylactic surgery is recommended at an early age. We present a case of the patient carrier of mutation of the proto-oncogene type RET with medullary thyroid carcinoma prophylactic surgery already in adulthood.Carcinoma medullary thyroid is a rare, malignant neoplasm. Affected patients can be hereditary carriers, but in most cases they are spontaneously affected. In hereditary cases, prophylactic surgery is recommended at an early age. We present a case of the patient carrier of mutation of the proto-oncogene type RET with medullary thyroid carcinoma prophylactic surgery already in adulthood.