Treating ‘osteoporosis’: a near miss in an unusual case of FGF-23-mediated hypophosphataemic osteomalacia

SUMMARY: We present the case of a 60-year-old female who developed repeated atraumatic stress fractures. She was initially diagnosed with osteoporosis based on her dual-energy X-ray absorptiometry (DXA) scan bone mineral density (BMD) T-scores and started on denosumab therapy. Secondary osteoporosis screen revealed abnormal myeloma screen and low serum phosphate levels. It was thought that the patient had multiple myeloma with associated Fanconi-related tubular dysfunction. However, fibroblast growth factor-23 (FGF-23) levels were grossly elevated, making Fanconi syndrome unlikely. The patient was subsequently diagnosed with two separate conditions, namely cardiac amyloid light-chain (AL) amyloidosis and FGF-23-related hypophosphataemia, likely due to tumour-induced osteomalacia. This case highlights the importance of excluding osteomalacia as a cause of low BMD and checking FGF-23 levels in the workup for hypophosphataemia. LEARNING POINTS: Tumour-induced osteomalacia is a difficult diagnosis as the tumour is often small and slow growing. Imaging may fail to identify a tumour, and treatment therefore consists of calcitriol and phosphate replacement. Tumour-induced osteomalacia should be suspected in the adult presenting with new-onset hypophosphataemia, elevated FGF-23 levels and isolated renal phosphate wasting. Serum phosphate is not part of the routine chemistry panels. Routinely checking phosphate levels prior to initiating antiresorptive therapy is warranted. DXA cannot distinguish low bone mineral density due to osteoporosis from osteomalacia. Antiresorptive therapy should be avoided in osteomalacia due to the risk of clinical and radiographic deterioration.