7191 When Pain Is Misdiagnosed: A Case of Parathyroid Carcinoma Masked as Fibromyalgia

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Abstract Disclosure: H. Pham: None. P.V. Kumar: None. S. Taylor: None. Parathyroid carcinoma is a rare and aggressive cause of hyperparathyroidism, typically associated with <1% of primary hyperparathyroidism cases. Incidence occurs mainly in the 5th decade of life, with less than 4% of all cases presenting in patients under age 30. Here, we highlight a case involving a 28-year-old woman with fibromyalgia who presented for worsening right shoulder pain in the setting of progressive, poorly characterized pain for the past year. Her initial physical exam was unremarkable aside from a non-tender left sided neck enlargement. However, her laboratory results were significant for corrected serum calcium of 15 mg/dL, serum phosphorus of 2.3 mg/dL, parathyroid hormone (PTH) >2500 pg/mL, and alkaline phosphatase 1604 units/L. X-ray of her right shoulder demonstrated mottled bone concerning for degenerative disease. Thyroid ultrasound revealed a complex cystic lesion posterior to the left thyroid lobe measuring 5.3 x 3.9 x 2.9 cm, worrisome for giant parathyroid adenoma. CT abdomen/pelvis showed diffuse osteosclerosis and innumerable lytic lesions, consistent with osteitis fibrosa cystica, as well as bilateral nephrolithiasis, likely medullary nephrocalcinosis. Her hypercalcemia was treated with aggressive hydration with normal saline and IV pamidronate. She underwent surgical resection of her parathyroid adenoma, which was unfortunately complicated by symptomatic hypocalcemia requiring prolonged care in the ICU. Her calcium levels did improve within a few days, and she was subsequently discharged with close outpatient follow-up. Final pathology report showed predominant thickened trabecular growth pattern with focal vascular invasion consistent with parathyroid carcinoma. Our patient’s case represents a rare presentation of parathyroid carcinoma in a young patient without any additional risk factors. Although atypical, her severe symptomatic hypercalcemia, nephrolithiasis, presence of large neck mass, and diffuse skeletal disease warranted immediate concern for malignant hyperparathyroidism instead of benign adenoma. Delayed diagnosis in our patient led to prolonged bone disease and severe complications. While hungry bone syndrome typically manifests post-parathyroidectomy in patients with end-stage renal disease, our patient experienced this complication in the setting of advanced, untreated hyperparathyroidism. We should consider hypercalcemia and hyperparathyroidism in our differential diagnosis for chronic pain, especially if pain remains uncontrolled with conservative measures. The most appropriate initial test for evaluation of hypercalcemia is a PTH level to distinguish between PTH-dependent and PTH-independent etiologies. Parathyroid carcinoma, even in young patients, should be considered when PTH is >3x upper limit of normal, serum calcium is >14, or skeletal and renal involvement is present. Presentation: 6/3/2024