Introduction: Neck US and Tc99m-MIBI are the most widely used method for preoperative localization of parathyroid adenoma. However, in the case of coincident thyroid tumor and PHPT, careful work-up with interpretation is needed. Here we report a case of coincident thyroid tumor and PHPT. Case: A 39-year-old woman without medical history except thyroid nodule visited our clinic for hyperglycemia. 3 years ago, thyroid nodule (2cm) was detected on ultrasound and fine needle aspiration cytology revealed benign nodular hyperplasia. Laboratory data showed: serum fasting glucose 125mg/dl, HbA1c 6.5%. Serum total calcium 12.1mg/dl, phosphorus 2.3mg/dl, PTH 105.42pg/ml, 25(OH)vit-D 24.8ng/ml, 24 hour urine calcium excretion 426mg/day. Bone mineral density was decreased (Z-score of lumbar spine -1.6). The results suggested diabetes mellitus and PHPT. Therefore, neck US and 10 mCi Tc-99m MIBI scan were performed. Neck US showed 2cm sized nodule in left thyroid that was previously diagnosed as benign and there was no abnormal lesion suggesting parathyroid adenoma. MIBI scan showed single hot uptake in left thyroid nodule, and intrathyroidial parathyroid adenoma was suspected. Lt lower thyroid and nodule was removed. However, pathologic result revealed minimally invasive follicular thyroid carcinoma. Despite surgery, hypercalcemia was not corrected. We performed follow-up MIBI scan and scan showed focal increased uptake below lower pole of right thyroid that was finally con. rmed parathyroid adenoma. Conclusion: Neck US and Tc99m-MIBI are most widely used for preoperative localization of parathyroid adenoma. However, thyroid cancer can result in false positive on MIBI scan. And neck US is sensitive method for thyroid and parathyroid mass, intrathyroidal parathyroid adenoma can be misdiagnosed. In conclusion, preoperative evaluation for PHPT should be carefully made in cases with concomitant thyroid mass.