SUN-822 - Parathyroid Carcinoma Masquerading as Adenoma: When Mild Biochemical Abnormalities Belie Malignancy

Introduction:

Parathyroid carcinoma is an exceptionally rare malignancy, accounting for less than 1% of primary hyperparathyroidism cases. It usually presents with markedly elevated calcium and parathyroid hormone (PTH) levels. Presentation with modest, adenoma-level biochemical abnormalities is uncommon, often delaying recognition and diagnosis.

Case Presentation:

A 20-year-old Caucasian male was evaluated for persistent hypercalcemia (10.1–10.7 mg/dL) and elevated PTH (100.5–133 pg/mL) over 12–18 months. Early referral was initially deferred due to mild elevations with vitamin D deficiency, but persistent abnormalities prompted surgical consultation. He reported chronic fatigue, brief lightheadedness, long bone pain, and thirst with dehydration, but denied polyuria, nephrolithiasis, GI symptoms, pancreatitis, muscle weakness, cognitive changes, or fractures. Past medical history included hypotension treated with midodrine, seasonal allergic rhinitis, and bronchitis. There was no personal or family history of endocrine tumors, parathyroid disorders, radiation exposure, or relevant medications.

Preoperative labs suggested mild primary hyperparathyroidism, more consistent with an adenoma. He underwent parathyroidectomy. Intraoperative PTH fell 30% after excision of the right upper parathyroid lesion, prompting continued exploration. Removal of a second right lower parathyroid lesion led to an 89% PTH decrease. Left glands appeared normal and were preserved.

Histopathology confirmed parathyroid carcinoma in the right upper gland (1.5 × 0.9 × 0.8 cm; 0.8 g) with angioinvasion, fibrosis, and infiltrative growth. Immunohistochemistry showed positive PTH, Bcl-2, MDM2, and Ki-67 < 5%. The right lower gland was a hypercellular adenoma without malignancy. Postoperatively, transient hypocalcemia required calcium supplementation. PTH and calcium normalized by one month.

Discussion/

Conclusion:

The most striking feature is the discordance between mild biochemical abnormalities and the ultimate diagnosis. Calcium never exceeded 10.7 mg/dL, and PTH was only 1.2–1.5-fold elevated, in contrast to typical presentations. Parathyroid carcinoma can mimic a benign adenoma biochemically and clinically, creating a diagnostic trap. Recognition requires high clinical suspicion, attention to persistent hypercalcemia, intraoperative findings, and meticulous histopathological evaluation, including immunohistochemistry. The coexistence of malignant and benign lesions underscores the need for longitudinal monitoring and thorough surgical assessment, even when classic severe features are absent.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*