Development of tertiary hyperparathyroidism after phosphate supplementation in oncogenic osteomalacia

Oncogenic osteomalacia is a rare paraneoplastic syndrome. It is characteriz ed by bone pain, muscle weakness, gait disturbance, fractures and skeletal deformities. Hypophosphatemia, diminished renal phosphate reabsorption, dec reased 1,25 dihydroxy Vitamin D and elevated alkaline phosphatase are the b iochemical hallmarks of this disorder. Most tumors are of mesenchymal origi n. We report the case of a 39-year-old woman with oncogenic osteomalacia ca used by osteosarcoma of the right scapula which was unrecognized for severa l years. She subsequently developed tertiary hyperparathyroidism after trea tment with oral phosphate and Vitamin D. This case illustrates that oncogen ic osteomalacia may persist for many years before the tumor is discovered. This is because the tumors are frequently very small and are in obscure loc ations. The uniqueness of this case is the coexistence of hyperparathyroidi sm and oncogenic osteomalacia. Five other cases have been reported up to da te. All patients had received phosphate supplement, ranging from 10 to 14 y ears prior to their diagnosis. Interestingly, our patient was on the treatm ent for only 2 years. The proposed mechanism is that exogenous phosphate st imulates parathyroid activity through sequestration of calcium. (C) 2000, E ditrice Kurtis.