Oncogenic Osteomalacia From a Primary Phosphaturic Mesenchymal Tumor of the Toe: A Case Report

Abstract Oncogenic osteomalacia is an acquired, rare paraneoplastic syndrome characterized by renal phosphate wasting and subsequent hypophosphatemic osteomalacia. The condition is usually associated with a phosphaturic mesenchymal tumor, which produces fibroblast growth factor 23, the primary circulating factor responsible for reduced tubular phosphate reabsorption. Clinically, adult patients typically present with bone pain, myalgia, recurrent and/or multiple stress fractures, and fatigue, with serum levels typified by low 1,25-(OH)2 vitamin D3 , increased alkaline phosphatase, and normal calcium, parathyroid hormone, calcitonin, 25-OH-vitamin D3 , and 25,25-(OH)2 vitamin D3 levels. The tumor in question is typically benign and can be of little clinical significance apart from its role in causing hypophosphatemic osteomalacia. Detection of the tumor, therefore, can often be delayed and requires an astute index of suspicion.