Selective blood sampling for FGF-23 in tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is caused by the hormone fibroblast growth factor 23 (FGF-23). It is mainly produced in the tissue of mesenchymal tumors. Patients with TIO frequently suffer from a chronic decompensated pain syndrome and/or muscle weakness with postural deformity. Despite the severity of the disease, the diagnosis is frequently established late. In some cases, it takes several years to establish the condition. This case report concerning a 68-year old woman demonstrates the selective blood sampling for FGF-23 as path-breaking diagnostics to confirm the diagnosis of a neuroendocrine tumor.

Tumor-induced osteomalacia is a rare condition compared to other paraneoplastic syndromes.It causes complex symptoms such as progressive reduction of physical capacity, exhaustion, fatigue, a decompensated pain syndrome of the musculoskeletal system and fractures of several bones.Elevated serum levels of FGF-23 implicate massive phosphate elimination and resulting hypophosphatemia.The diagnosis is often established over a period of several years because the localization of small FGF-23-producing tumors is complicated.It is the combination of MRI and selective blood sampling for FGF-23 which permits reliable identification of tumors causing TIO and leads to accurate localization.In a patient with generalized pain and reduced physical capacity, osteological parameters such as phosphate, 25-OH vitamin D₃ and 1,25-(OH)₂D₃, as well as bone-specific alkaline phosphatase levels in serum should be determined. Hypophosphatemia should always lead to further diagnostic investigations aiming at the detection of an FGF-23-producing tumor.