What is the mechanism of iron-induced hypophosphatemia?¶
Answer: IV iron — chiefly ferric carboxymaltose (FCM) — causes hypophosphatemia by driving FGF23-mediated renal phosphate wasting.
- FCM infusion acutely raises circulating FGF23 3- to 6-fold. FGF23, along with 1,25-dihydroxyvitamin D and PTH, is one of the main regulators of serum phosphate.
- The FGF23 spike reduces proximal tubular reabsorption of filtered phosphate, causing inappropriate urinary phosphate excretion. Phosphate typically reaches its nadir about 2 weeks after the initial infusion.
- FGF23 also lowers 1,25-dihydroxyvitamin D, which decreases gut phosphate absorption and serum calcium, secondarily raising PTH.
- Because PTH also increases renal phosphate excretion, a second wave of PTH-mediated wasting sustains the hypophosphatemia beyond the initial period of FGF23 elevation.
Risk factors: FCM above all; also normal kidney function, more severe iron deficiency, lower body weight, baseline hypophosphatemia, and repeated IV iron doses.
Symptoms are fatigue and myalgia, which overlap with the symptoms of iron deficiency itself — so prevention, by choosing another formulation, beats trying to detect it.
Treatment is difficult: the mechanism is renal wasting, so supplementation is largely excreted rather than retained.
Selected trials
| Study | Design | Key finding |
|---|---|---|
| Magagnoli, AJH 2025 | Systematic review | 50–92% (FCM) vs 2–8% with other formulations |
| Richards, JAMA Open 2025 | RCT, surgical patients | 26% (FCM) vs 4.5% (placebo); longer stay, 12 vs 9 days |
| Wolf, JAMA 2020 | RCT | 75% (FCM) vs 8% (FD) |
| Wolf, JCI 2018 | RCT | 50% (FCM) vs 1% (ferumoxytol) |
| Zoller, Gut 2023 | RCT, IBD | 51% (FCM) vs 8% (FD) |
FCM = ferric carboxymaltose; FD = ferric derisomaltose.