Anti-PHEX autoantibodies were detected using both luciferase immunoprecipitation systems and fl ow cytometry, underscoring the importance of complementary methods for detecting antibodies against native conformational epitopes.<h4>Conclusions</h4>Recognition of AIO should be particularly considered in patients with acquired FGF23-related hypophosphatemia who have undetectable PMTs, relatively mild disease activity, and concurrent autoimmune diseases. The gene discussed is FGF23; the disease is autoimmune disease.