Keyword: Fibroblast Growth Factor 23

ABSTRACT: Post-bariatric surgery patients face higher risk of micronutrient malabsorption and often receive intravenous iron like Ferric carboxymaltose (FCM).  Recent studies associate FCM with severe (serum phosphate levels < 1 mg/dL) and prolonged hypophosphatemia caused by elevated Fibroblast Growth Factor 23 (FGF23) levels, which reduces phosphate reabsorption in the kidneys via α-Klotho co-receptor. FCM, in particular, can therefore lead to notable and sustained FGF23-mediated hypophos-phatemia. We present a case of prolonged and severe hypophosphatemia secondary to fibroblast growth factor 23 (FGF23)-mediated phosphaturia following ferric carboxymaltose (FCM) administration in a post-bariatric surgery patient. This case underscores the complex interplay between intravenous iron therapy, phosphate metabolism, and altered absorption physiology following bariatric surgery. It highlights the need for clinicians to monitor phosphate levels after FCM administration, particularly in high-risk patients such as those with malabsorption.