Kayes M1,2, Aravindan A1, Wong J1
1Department of Renal Medicine, Liverpool Hospital, Sydney, Australia, 2University of New South Wales, Sydney, Australia
Background: Fibroblast growth factor 23 (FGF-23) is an essential protein in phosphate homeostasis. FGF-23 is produced primarily in bone osteocytes as a response to elevated serum phosphate or 1,25-Hydroxy Vitamin D (1,25(OH)2D). FGF-23 acts on the kidney by preventing tubular reabsorption of phosphate and reduction of serum 1,25(OH)2D. This also results in hypocalcaemia with an elevation of serum parathyroid hormone (PTH).
Case Report: A 39 year old male was admitted with an asthma exacerbation. During his admission he was found to have an incidental rapid decline in his serum phosphate levels to a nadir of 0.29mmol/L, despite adequate replacement. He also developed a concurrent hypocalcaemia with corrected calcium levels dropping to 1.93mmol/L. The patient had excellent dietary intake and no history of refeeding.
Investigations revealed normal renal function with a serum creatinine of 62µmol/L with an estimated glomerular filtration rate greater than 90ml/min/1.73m². His 25-Hydroxy Vitamin D levels were stable at 53nmol/L. His serum PTH was elevated at 14.7pmol/L. His fractional excretion of phosphate was significantly elevated at 63.09%.
Review of medications revealed he received an intravenous iron polymaltose infusion 5 days prior to declining phosphate levels. A serum FGF-23 level was elevated at 155ng/L (upper-limit 95.4ng/L). A review of previous admissions revealed identical patterns of incidental hypophosphatemia days after polymaltose administration.
Conclusions: Iron infusion related hypophosphataemia has been reported previously, most commonly occurring after administration of smaller molecular-weight preparations (carboxymaltose, sucrose). The mechanism resulting in FGF-23 elevation is still undetermined with main theories suggesting either an effect on osteocytes increasing production or preventing proteolytic cleavage and inhibition. While rare, hypophosphataemia secondary to polymaltose should be considered in patients with normal renal function.
Masnun Kayes is currently a renal advanced trainee within the South West Sydney and Central Network in New South Wales.