In Search of a Missing Adenoma

Background: The most common cause of primary hyperparathyroidism (PHPT) is overproduction of PTH by a parathyroid gland adenoma. While definitive therapy is parathyroidectomy, 4% of patients develop persistent PHPT - a sustained hypercalcemic state that is detected within six months of parathyroidectomy. A missed parathyroid adenoma is the most common cause of persistent PHPT, and accurately locating these glands presents a diagnostic challenge. We describe a rare case of persistent PHPT due to a missed mediastinal parathyroid adenoma. Case: A 54-year-old woman with a history of PHPT presented with abdominal pain, nausea, and decreased oral intake. She underwent parathyroidectomy six months ago with reimplantation of one parathyroid gland into the right sternocleidomastoid muscle (SCM). She was now hypercalcemic to 13.9 mg/dL (8.5–10.5) with intact PTH 1273 pg/mL (15.0–65.0), vitamin D 25-OH 31.4 ng/mL (>30.0), and normal PTHrP. She was not taking calcium, and other causes of hyperparathyroidism were excluded. Sestamibi scintigraphy localized only to the right SCM, and the initial impression was recurrent HPT due to the previously implanted gland. Follow-up CT neck with and without contrast failed to localize any regrowth in the SCM, but did reveal a 1.4 cm mediastinal soft tissue mass, suspicious for an ectopic parathyroid adenoma. She subsequently underwent video-assisted thoracoscopic excision, and pathology was consistent with ectopic hypercellular parathyroid tissue. Post-operatively, her PTH down-trended and calcium normalized. Conclusion: This case describes a small yet biochemically aggressive mediastinal adenoma causing persistent PHPT. While sestamibi scans have ~90% sensitivity for localization of ectopic adenomas, they can fail to identify a small culprit lesion in 12% of patients, whereas CT imaging with and without contrast has increased sensitivity for adenomas <2 cm. Thus, diagnosing persistent PHPT requires sestamibi scan in combination with other imaging modalities for accurate diagnosis of missed adenomas.