Background <p>Clinically suspected parathyroid adenomas, which are not localized by conventional imaging such as ultrasonography and technetium-99&#xa0;m (99mTc) sestamibi scintigraphy, remain challenging. We describe two patients with hypercalcemia and elevated parathyroid hormone (PTH) levels, in whom ultrasonography or technetium-99&#xa0;m sestamibi scintigraphy failed to identify the lesions. Both were successfully localized by novel combined use of fluorine 18 (¹⁸F) fluorocholine positron emission tomography/computed tomography (FCH PET/CT) in conjunction with parathyroid venous sampling (PVS).</p> Case presentation <p>In the first case of 59 year-old Korean man with diabetes, a 2.0-cm mass in the left mediastinum was identified and removed via thoracoscopic robotic excision, with histopathology confirming an ectopic parathyroid adenoma. Postoperatively, serum calcium, PTH and creatinine levels improved, and the patient’s daily insulin requirement decreased from 90 to 15 units. In the second case of 55 year-old Korean woman, a 0.8-cm nodule was localized in the retropharyngeal space posterior to the right pyriform sinus. Due to the deep location and high surgical risk, the patient was managed non-surgically with annual intravenous bisphosphonate to control hypercalcemia and prevent bone loss.</p> Conclusions <p>These cases highlight the diagnostic value of FCH PET/CT and PVS for precise localization of parathyroid adenomas not detectable by first-line imaging, enabling optimal management and potentially improving outcomes beyond calcium and bone metabolism, including renal function and glucose homeostasis. To the best of our knowledge, this is the first case report demonstrating the clinical advantage of the combined use of FCH PET/CT and PVS.</p> Clinical trial number <p>Not applicable</p>