Primary hyperparathyroidism due to ectopic parathyroid adenomas can pose diagnostic and management challenges, especially when imaging studies have localised the lesions to different sites. We report a case of symptomatic hypercalcaemia due to a mediastinal parathyroid adenoma. Ultrasonography identified a nodule posterior to the right thyroid gland. However, computed tomography and technetium-99m sestamibi scintigraphy revealed an ectopic parathyroid adenoma located in the anterior mediastinum. The adenoma was successfully removed through a median sternotomy. However, postoperatively, the patient developed prolonged symptomatic hypocalcaemia, possibly due to suppression of the normal parathyroid gland function, although the presence of concomitant hungry bone syndrome was possible. The histopathology of the mediastinal mass was consistent with a parathyroid adenoma.