A case of FGF23-related disease treated as Fanconi syndrome induced by Sjögren’s syndrome

Abstract

A 43-year-old woman with Sjögren’s syndrome had developed hypophosphatemia six years previously. The patient had been diagnosed with Fanconi syndrome induced by Sjögren’s syndrome and was treated with oral phosphate and active vitamin D. Despite supplementation, severe hypophosphatemia continued. The serum fibroblast growth factor 23 (FGF23) level was 32.7 pg/ml three years previously, above the 30 pg/ml diagnostic cut-off for FGF23-related disease. However, the result was interpreted as a deviation rather than a diagnosis. The patient needed higher doses of oral phosphate, and she experienced bilateral ankle pain and difficulty walking. One year previously, the FGF23 level was 53.7 pg/ml, which confirmed the diagnosis of FGF23-related disease. Although tumor-induced osteomalacia was strongly suspected due to its acquired onset, a causative tumor had not been found. The measurement of FGF23 level is crucial for the evaluation of musculoskeletal symptoms and phosphaturic hypophosphatemia. FGF23-related disease, such as tumor-induced osteomalacia, presents several inherent challenges for accurate diagnosis.