SAT-760 A Severe and Uncommon Manifestation of Primary Hyperparathyroidism

Abstract Disclosure: S.D. Leungsuwan: None. Background: Primary hyperparathyroidism (PHPT) is a common endocrine disorder increasingly diagnosed at the asymptomatic stage. We present a case of symptomatic PHPT with advanced manifestations. Clinical case: A 46-year-old female presented to the emergency department for a painless, gradually enlarging neck lump accompanied by intermittent epigastric discomfort and constipation. No constitutional or infective symptoms were present. Other than a history of nephrolithiasis, there is no other past medical history, no long-term medication, traditional medicine, or supplements. Clinical examination revealed a large, lobulated central neck mass; Pemberton’s sign was negative with no cervical adenopathy. Systemic examination was unremarkable. Biochemical investigations showed adjusted serum calcium 3.78mmol/L (2.10-2.60), serum phosphate 0.80mmol/L (0.65-1.65), serum magnesium 0.61mmol/L (0.65-0.95), serum creatinine 103umol/L (50-90), serum intact parathyroid hormone (iPTH) 253pmol/L (1.30-7.60), and 25-hydroxyvitamin D 29.3ug/L (30.0-100.0). She was treated with aggressive hydration, subcutaneous calcitonin, and two doses of intravenous zoledronic acid 4mg; her serum calcium improved but plateaued at 3.00-3.10mmol/L. Computed Tomography (CT) of the neck characterized a heterogeneous mass in the left neck corresponding to a region of increased uptake with central photopaenia on the parathyroid uptake scan. CT imaging of the thorax, abdomen, and pelvis also noted scattered obstructive nephrolithiasis and osteitis fibrosa cystica (OFC). She was offered but declined genetic studies for PHPT. She underwent en-bloc resection of the left hemi-thyroid, left parathyroid, and pre-tracheal nodes. Intraoperative iPTH fall satisfied the Miami criterion. Histology showed a completely resected parathyroid neoplasm with an intact capsule and central cystic change, with no evidence of metastatic carcinoma. Post-operatively, the patient developed hungry bone syndrome, requiring calcium infusions and high enteral doses of calcium carbonate and calcitriol to maintain normal calcium and phosphate levels. She was planned to be closely followed up to titrate calcium carbonate and calcitriol levels, monitor for recurrence of PHPT with continued suspicion for parathyroid carcinoma, follow-up for complications of hyperparathyroidism and post-operative hypoparathyroidism, and genetic studies. Discussion: This is an atypical case of PHPT presenting with a large neck mass with an astronomical level of iPTH, hypercalcaemia refractory to bisphosphonates and medical therapy, and OFC as manifestations of PHPT. While histologic features appeared benign, a high index of suspicion must be maintained for parathyroid carcinoma. Presentation: Saturday, July 12, 2025