Multiple Brown Tumors Secondary to Parathyroid Carcinoma: A Challenging Diagnosis

Parathyroid carcinoma is an extremely rare endocrine neoplasm that accounts for less than 1% of the cases of primary hyperparathyroidism (PHPT). Continuous exposure to high levels of parathyroid hormone (PTH) induces an increase in bone remodeling and patients may present with osteitis fibrosa cystica, which is characterized by subperiosteal resorption of the phalanges, diffuse osteopenia, salt and pepper appearance of the skull, bone cysts, and brown tumors. Brown tumors occur in less than 5% of all patients with any form of hyperparathyroidism. Due to similar clinical, radiographic, and histological appearance, differential diagnosis of brown tumors includes primary and secondary bone tumors. We report a case of a 67-year-old female diagnosed with multiple osteolytic lesions initially thought to be bone metastasis of thyroid carcinoma. Further work-up led to the diagnosis of brown tumors due to parathyroid carcinoma. We want to emphasize the inclusion of osteitis fibrosa cystic in the differential diagnosis of osteolytic lesions and the need to perform serum calcium and PTH measurements when investigating these lesions.