Tumor-induced osteomalacia (TIO) is a paraneoplastic disorder characterized by
hypophosphatemia,
phosphaturia, inappropriately low serum levels of
1,25-dihydroxyvitamin D for
hypophosphatemia, and skeletal undermineralization. Patients with TIO suffer from severe
muscle weakness and
pain. Because surgical removal of the responsible
tumors is the only satisfactory treatment for TIO, identification of the
tumors is clinically essential. However, because they are predominantly slow-growing
neoplasms of benign mesenchymal origin, localization of the responsible
tumors is often very difficult. Moreover, even if a
tumor is found in a patient with hypophosphatemic
osteomalacia, we have had no way to know that the
tumor is actually causing the disease.
Fibroblast growth factor-23 (FGF-23) was recently identified as a causative factor for TIO and was shown to induce renal
phosphate wasting. We have recently shown that the circulatory FGF-23 level was high in a patient with TIO and rapidly decreased after removal of the responsible
tumor. For the first time, we describe a patient with adult-onset hypophosphatemic
osteomalacia in whom a clinical diagnosis of TIO was confirmed before surgical removal of the
tumor by localizing the responsible
tumor using venous sampling for FGF-23 together with magnetic resonance imaging. This combinatorial procedure would be clinically useful for sporadic cases of
hypophosphatemic rickets/
osteomalacia.