Medullary thyroid carcinoma (MTC) is derived from the parafollicular or C-cells. As surgical resection is the only curative therapy for MTC, the early diagnosis is important for the patient's survival. F18-Fluorodeoxyglucose positron emission tomography-computed tomography (F-18 FDG PET CT) is a noninvasive imaging method which can be used to diagnose malignant thyroid tumors including recurrent or residual MTC. However, due to the limitations of this technique, it is difficult to differentiate benign from malignant thyroid tumors. We herein present a 47-year-old woman with ovarian cancer history who was found to have thyroid incidentaloma with metastatic cervical lymph node through F-18 FDG PET CT. Thyroid incidentaloma of the patient was examined by fine needle aspiration and the result of this diagnostic procedure showed suspicious MTC. The patient was subsequently diagnosed as having sporadic medullary thyroid carcinoma and metastatic cervical lymph node due to ovarian cancer.