This case report highlights a rare presentation of parathyroid carcinoma in a 28-year-old woman who initially presented with worsening right shoulder pain and progressive, poorly characterized pain over the past year. Her initial physical examination was unremarkable, but laboratory results showed significantly elevated serum calcium, phosphorus, parathyroid hormone (PTH), and alkaline phosphatase levels. Imaging revealed mottled bone in the right shoulder and a large cystic lesion in the thyroid, suggesting a giant parathyroid adenoma. Further imaging showed diffuse osteosclerosis and lytic lesions, consistent with osteitis fibrosa cystica, and bilateral nephrolithiasis. The patient was treated with aggressive hydration and IV pamidronate, followed by surgical resection of the parathyroid adenoma. Post-surgery, she experienced symptomatic hypocalcemia, requiring ICU care. The final pathology report confirmed parathyroid carcinoma. This case underscores the importance of considering hypercalcemia and hyperparathyroidism in the differential diagnosis of chronic pain, especially when conservative measures fail to control pain. Early diagnosis and treatment are crucial to prevent severe complications.This case report highlights a rare presentation of parathyroid carcinoma in a 28-year-old woman who initially presented with worsening right shoulder pain and progressive, poorly characterized pain over the past year. Her initial physical examination was unremarkable, but laboratory results showed significantly elevated serum calcium, phosphorus, parathyroid hormone (PTH), and alkaline phosphatase levels. Imaging revealed mottled bone in the right shoulder and a large cystic lesion in the thyroid, suggesting a giant parathyroid adenoma. Further imaging showed diffuse osteosclerosis and lytic lesions, consistent with osteitis fibrosa cystica, and bilateral nephrolithiasis. The patient was treated with aggressive hydration and IV pamidronate, followed by surgical resection of the parathyroid adenoma. Post-surgery, she experienced symptomatic hypocalcemia, requiring ICU care. The final pathology report confirmed parathyroid carcinoma. This case underscores the importance of considering hypercalcemia and hyperparathyroidism in the differential diagnosis of chronic pain, especially when conservative measures fail to control pain. Early diagnosis and treatment are crucial to prevent severe complications.