8557 Osteitis Fibrosa Cystica as the Initial Manifestation of Severe Primary Hyperparathyroidism: A Case Report

Abstract Disclosure: L.A. Robles Gomez: None. G. Gill: None. A. Vincent, DO: None. A. Mahmoud, MD: None. Introduction: Osteitis fibrosa cystica (OFC) is a pathognomonic yet uncommon finding of hyperparathyroidism seen in less than 2% of patients with primary hyperparathyroidism in Western countries. We present a case of infrequent finding of late-stage severe hyperthyroidism with asymptomatic hypercalcemia and incidental findings of osteolytic bone lesions. Clinical Case: A 28-year-old female with a history of PCOS, fatigue, and chronic abdominal pain presents to the clinic after recent work-up during an ED visit demonstrated incidental findings of osteolytic bone lesions in the setting of mild hypercalcemia. Physical examination was relatively unremarkable except for mild tenderness to palpation on the right lower quadrant. Laboratory significant calcium 10.7 mg/dl (8.6-10.2 mg/dL), albumin 3.6 g/dL, Creatinine 0.56 mg/dL, GFR 127 mL/min, Alkaline phosphatase 187 IU/L (37-127 IU/L), AST 24 IU/L, ALT 45 IU/L, Intact PTH and calcium: PTH 413 pg/mL (15-65 pg/mL), Calcium 11.5 (8.7-10.4 mg/dL), Bone Specific Alkaline Phosphatase: 54.5 mcg/L (4.7-17.8 mcg/L), Phosphorus 2.1 mg/dL (2.4-4.8 mg/dL), Magnesium 1.7 mg/dL (1.5-2.5 mg/dL), Vitamin D 25 10 mg/mL (20-100 ng/mL), TSH 2.12 uIU/mL (0.46-5.05 uIU/mL). CT abdomen pelvis w/o contrast with multiple predominantly lytic expansile bone lesions, mainly in the pelvis, with the most extensive 6.2 cm lesion in the superior, posterior right ilium, the anterior acetabulum, and the posterior right ischium. A prior CT done in 2018 showed a similar distribution of lytic lesions. The patient denied any previous personal or family history of hypercalcemia or parathyroid abnormalities despite a review of prior outside records revealed abnormal labs dating back to 2018 with intact PTH 406 pg/mL, calcium 10.3 mg/dL, Ionized calcium 10.7 mg/dl (3.30-6.0 mg/dL), albumin 3.3g/dL, alkaline phosphatase 288 IU/L. The patient denied prior symptomatology of hypercalcemia, including renal stones, or prior treatment for hypercalcemia. Further work-up was ordered, and the patient was referred to endocrine surgery for management. Conclusion: Osteitis fibrosa cystica (OFC) has become a rare manifestation of primary hyperparathyroidism (PHPT) due to early diagnosis of hypercalcemia by routine lab monitoring. Skeletal findings typically seen on PHPT can vary from bone reabsorption-acroosteolysis, subchondral bone resorption, brown tumors, salt-and-pepper skull, and osteopenia. Subchondral reabsorption around specific joints commonly affects pubic symphysis and sacroiliac joints, as seen in our patient. The mechanism is due to PTH-mediated osteoclastic absorption of bone and surrounding cartilage. Due to the rare presentation of PHPT with associated osteitis fibrosa cystica, diagnosis requires a high degree of clinical suspicion, as delayed diagnosis can lead to avoided manifestation, as described by our report. Presentation: 6/3/2024