osteitis fibrosa cystica

Osteitis fibrosa cystica results from chronically elevated parathyroid hormone (PTH), which stimulates osteoclastic bone resorption via RANKL upregulation on osteoblasts, leading to increased osteoclast activity and bone turnover. This causes replacement of normal bone marrow with fibrous tissue, cystic degeneration, and hemorrhagic deposits called 'brown tumors' (due to hemosiderin deposition from microfractures and hemorrhage). Radiographically, classic findings include subperiosteal bone resorption (especially of the radial aspect of the middle phalanges), a 'salt-and-pepper' appearance of the skull, bone cysts, and tapering of the distal clavicles. Clinically, patients present with bone pain, pathologic fractures, and skeletal deformities, often accompanied by other signs of hypercalcemia ('bones, stones, groans, psychiatric overtones'). It is classically associated with primary hyperparathyroidism (due to parathyroid adenoma, hyperplasia, or carcinoma) but can also occur in severe secondary hyperparathyroidism, particularly in the setting of chronic kidney disease (renal osteodystrophy), where hyperphosphatemia and hypocalcemia drive compensatory PTH elevation. Laboratory findings typically show elevated PTH, elevated calcium (in primary disease) or low/normal calcium (in secondary disease), elevated alkaline phosphatase, and elevated urinary cAMP. Treatment involves correcting the underlying cause—parathyroidectomy for primary hyperparathyroidism or managing CKD-related mineral bone disease (phosphate binders, vitamin D analogs, calcimimetics) for secondary hyperparathyroidism. Histologically, the lesions show increased osteoclasts in Howship's lacunae, fibrous stroma replacing marrow, and hemosiderin-laden macrophages giving the brown color to brown tumors.