Oral phosphate supplements reverse skeletal muscle abnormalities in a case of chronic fatigue with idiopathic renal hypophosphatemia.

A 57-yr-old man presented with a long history of undiagnosed fatigue but no evidence of bone disease. He was noted to have hypophosphatemia due to an idiopathic phosphaturia. Marked abnormalities of exercising skeletal muscle detected by phosphorus magnetic resonance spectroscopy and by plasma metabolite measurements were consistent with mitochondrial dysfunction. Oral phosphate supplements restored plasma phosphate concentration and muscle biochemistry to normal and produced considerable improvement in symptoms and exercise tolerance, although the phosphate concentration in muscle was only marginally low and increased little by treatment. We conclude that hypophosphatemia should be excluded in unexplained fatigue.