Parathyroid carcinoma is a rare and aggressive cause of hyperparathyroidism, accounting for less than 1% of primary hyperparathyroidism cases. It typically affects individuals in their 5th decade of life, with less than 4% of cases occurring in those under 30. This case describes a 28-year-old woman with fibromyalgia who presented with worsening right shoulder pain and progressive, poorly characterized pain over the past year. Her laboratory results showed severe hypercalcemia (15 mg/dL), elevated PTH (>2500 pg/mL), and other abnormalities. Imaging revealed a complex cystic lesion in the left thyroid area and diffuse osteosclerosis, consistent with osteitis fibrosa cystica and bilateral nephrolithiasis. She was treated with aggressive hydration and pamidronate, followed by surgical resection of a parathyroid adenoma. However, she developed symptomatic hypocalcemia requiring ICU care. Pathology confirmed parathyroid carcinoma. This case highlights the importance of considering parathyroid carcinoma in young patients with severe hypercalcemia, nephrolithiasis, and skeletal involvement, even in the absence of risk factors. Delayed diagnosis led to prolonged bone disease and complications. Hypercalcemia and hyperparathyroidism should be considered in the differential diagnosis of chronic pain, especially when pain is unresponsive to conservative treatment. The initial test for hypercalcemia should be PTH levels to distinguish between PTH-dependent and PTH-independent causes. Parathyroid carcinoma should be considered when PTH is >3x the upper limit of normal, serum calcium is >14, or skeletal and renal involvement is present.Parathyroid carcinoma is a rare and aggressive cause of hyperparathyroidism, accounting for less than 1% of primary hyperparathyroidism cases. It typically affects individuals in their 5th decade of life, with less than 4% of cases occurring in those under 30. This case describes a 28-year-old woman with fibromyalgia who presented with worsening right shoulder pain and progressive, poorly characterized pain over the past year. Her laboratory results showed severe hypercalcemia (15 mg/dL), elevated PTH (>2500 pg/mL), and other abnormalities. Imaging revealed a complex cystic lesion in the left thyroid area and diffuse osteosclerosis, consistent with osteitis fibrosa cystica and bilateral nephrolithiasis. She was treated with aggressive hydration and pamidronate, followed by surgical resection of a parathyroid adenoma. However, she developed symptomatic hypocalcemia requiring ICU care. Pathology confirmed parathyroid carcinoma. This case highlights the importance of considering parathyroid carcinoma in young patients with severe hypercalcemia, nephrolithiasis, and skeletal involvement, even in the absence of risk factors. Delayed diagnosis led to prolonged bone disease and complications. Hypercalcemia and hyperparathyroidism should be considered in the differential diagnosis of chronic pain, especially when pain is unresponsive to conservative treatment. The initial test for hypercalcemia should be PTH levels to distinguish between PTH-dependent and PTH-independent causes. Parathyroid carcinoma should be considered when PTH is >3x the upper limit of normal, serum calcium is >14, or skeletal and renal involvement is present.