Iron Infusion and Phosphate Drop: underestimated, but manageable
Some iron infusions temporarily lower the phosphate level through the hormone FGF23. How common this is, who carries a higher risk and how to keep it well in view through preparation choice and monitoring.
An iron infusion can be well tolerated. One accompanying effect is often overlooked: some preparations can lower the phosphate level in the blood. This effect is well studied, usually mild and temporary, and above all it is manageable. That is exactly what this article is about.
This article looks specifically at a single side effect of the iron infusion: the phosphate drop, technically called hypophosphatemia. You will find the broad overview of all reactions and of the differences between older and modern preparations in the article on the side effects of older and modern preparations. The complete overview of diagnostics, ferritin and indication is given by the pillar article on iron deficiency and iron infusions. Here we go one level deeper into phosphate.
The phosphate drop is no argument against the iron infusion. It is an argument for a deliberate procedure: the right preparation for the right person, with the right eye on the values. Whoever knows the effect can recognize it early and place it well, instead of being taken by surprise by it.
What happens in the body: the hormone FGF23
Many people with iron deficiency know the feeling of looking forward to an infusion and at the same time wondering what it does to the body. With phosphate a closer look is worthwhile, because the mechanism is surprisingly elegant and explains well why not every preparation reacts the same way.
At the center is a hormone with the cumbersome name fibroblast growth factor 23, or FGF23 for short. It is formed mainly in the bone and is a kind of regulator for the phosphate balance. Put simply, FGF23 sends the kidney the signal: excrete more phosphate. In addition it slows the activation of vitamin D, which further dampens phosphate uptake in the gut.
Normally FGF23 is not only formed, but also continuously broken down into smaller, inactive fragments. This is where iron comes into play. With iron deficiency the bone produces more FGF23, but at the same time breaks it down more strongly, so that the active portion often stays balanced. Some intravenous iron preparations appear to temporarily inhibit this breakdown step. The result: the proportion of active, intact FGF23 rises briefly, the kidney excretes more phosphate, and the level in the blood can fall.
Coppolino and colleagues describe in 2019 how intravenous iron intervenes in the phosphate balance through FGF23.
Key point: Certain iron formulations, especially ferric carboxymaltose, can trigger an increased phosphate loss through the kidney via a change in FGF23 metabolism. The overview explains why the preparations behave differently here.
Coppolino G et al. Ther Apher Dial. 2019;24(3):258-264. DOI: 10.1111/1744-9987.13435The phosphate drop is not a random intolerance. It follows an understandable hormonal path. Precisely because the mechanism is known, the effect is predictable and therefore manageable, instead of being an incalculable surprise.
How common the phosphate drop really is
Here it becomes concrete, and here the most important point of the whole topic shows itself: the frequency depends enormously on the preparation. A blanket statement like "iron infusions lower the phosphate level" falls clearly short.
The most extensive analysis so far comes from Schaefer and colleagues. They summarized 42 clinical studies and compared ferric carboxymaltose with iron isomaltoside, which today is called ferric derisomaltose.
Schaefer and colleagues systematically evaluated 42 clinical studies on phosphate drop after intravenous iron in 2020.
Key point: A phosphate drop occurred after ferric carboxymaltose in around 47 percent of those treated, after iron isomaltoside in only about 4 percent. The mean phosphate drop was also clearly larger with ferric carboxymaltose. In a portion of those treated the value stayed low for up to three months.
Schaefer B et al. Br J Clin Pharmacol. 2020;87(5):2256-2273. DOI: 10.1111/bcp.14643These figures gained additional weight through a very clean study: two parallel and identically designed, randomized studies from the USA that made exactly this question the main outcome.
Wolf and colleagues compared ferric derisomaltose with ferric carboxymaltose in two identically designed randomized studies in iron deficiency anemia in 2020.
Key point: A phosphate drop occurred with ferric carboxymaltose in 75.0 and 73.7 percent, with ferric derisomaltose in only 7.9 and 8.1 percent. The difference was very clear in both studies. The authors emphasize at the same time that the clinical significance of this difference needs to be researched further.
Wolf M et al. JAMA. 2020;323(5):432-443. DOI: 10.1001/jama.2019.22450The decisive sentence is not "iron infusions lower phosphate", but "some preparations do it much more often than others". That turns a supposed weakness of the method into a deliberate decision.
Who carries a higher risk
Not every person reacts the same. From the data, patterns can be read of which constellations make the phosphate drop more likely. These patterns help to look more closely before the dose.
Factors that can increase the risk
- Pronounced iron deficiency. Very low ferritin and a low transferrin saturation were associated with a higher risk in the meta-analysis.
- Normal kidney function. Paradoxical, but consistent in the data: precisely with good kidney function the phosphate loss carries more weight.
- Certain preparations. Ferric carboxymaltose much more often than ferric derisomaltose.
- Repeated or high doses. Several doses over time can strengthen or prolong the effect.
- After bariatric surgery. Here a particularly high risk showed itself in a prospective study.
- Chronic inflammatory bowel disease. Here too the effect was well documented.
Schoeb and colleagues investigated the phosphate course after ferric carboxymaltose in people with previous bariatric surgery in 2020.
Key point: 29 percent developed a new phosphate drop, accompanied by a measurable rise in intact FGF23 and a fall in active vitamin D. The authors recommend checking the phosphate value in this group after the dose.
Schoeb M et al. Obes Surg. 2020;30(7):2659-2666. DOI: 10.1007/s11695-020-04544-xFrom an integrative perspective I find this finding on kidney function especially instructive. It shows that a laboratory value always has to be read in context. The same infusion can have different effects in two people, depending on the starting situation. This is one perspective among several, and it does not replace careful medical assessment, but underlines it.
Which symptoms are possible and why they are easily overlooked
Many people with a mild phosphate drop notice nothing of it. The value falls, settles again, and the body copes with it. That is exactly the most common course. It becomes more difficult when the drop is more marked or lasts longer.
Then symptoms can appear that are insidiously unspecific: fatigue, muscle weakness, reduced resilience. The problem here is obvious. Exactly these symptoms are also expected with an iron deficiency. Whoever is still exhausted after the infusion easily attributes that to the iron, while in the background a low phosphate value plays along.
If the exhaustion after an iron infusion does not want to give way, sometimes a second thought is worthwhile. Not every lasting fatigue is residual iron deficiency. A lowered phosphate level can produce a similar picture and can be clarified with a simple blood check.
In rare cases, above all with repeated doses over a long time, a persistently low phosphate level can impair bone metabolism. In the specialist literature, individual courses with bone pain up to a softening of the bone are described. Such cases are the rare exception, not the rule, and they typically concern people who received infusions with a higher-risk preparation again and again over a long time.
Klein and colleagues describe in 2018 a person with Crohn's disease and chronic iron requirement who developed a severe, long-lasting hypophosphatemia after several doses of ferric carboxymaltose.
Key point: An elevated FGF23 and a high phosphate loss through the kidney led to a softening of the bone with bone pain. Only after stopping the preparation and targeted phosphate administration did the condition improve over months. The authors urge keeping this rare complication in view with ongoing iron requirement.
Klein K et al. BMJ Case Rep. 2018;2018:bcr-2017-222851. DOI: 10.1136/bcr-2017-222851How the phosphate drop can be managed
Now to the actual core that gives this topic its optimistic undertone. The phosphate drop is not only known, it can also be influenced. There are several points where you can intervene.
Levers for a deliberate procedure
- Preparation choice. The biggest lever. Where risk plays a role, a preparation with a lower phosphate tendency can be sensible. More on this in the article on the older and modern preparations.
- Assess the risk beforehand. Very low ferritin, good kidney function, planned multiple doses or a state after bariatric surgery speak for more attention.
- Check phosphate. Where it is sensible, the value can be measured simply in the blood after the dose, especially before further infusions.
- Take symptoms seriously. If the weakness lasts after the infusion, the phosphate value belongs on the list of things to check.
- Consider dose and intervals. With ongoing iron requirement the overall strategy counts, not the single infusion.
Teh and colleagues describe in 2020 a person with repeated admissions for unspecific symptoms that were at first attributed to the iron deficiency.
Key point: After several doses of ferric carboxymaltose a severe hypophosphatemia with elevated FGF23 and renal phosphate loss showed itself. The authors recommend checking the phosphate value routinely with parenteral iron, especially with carboxymaltose and polymaltose, since the complication is underdiagnosed.
Teh KK et al. Eur J Case Rep Intern Med. 2020;7(11):001860. DOI: 10.12890/2020_001860The phosphate drop is a good example of the core idea of a correctly performed iron infusion: it is not about the question whether, but about with what, in whom and with which eye on the values. Precisely this care makes the difference.
From my integrative perspective a final thought is added. Phosphate does not stand alone, but in a finely tuned web with calcium, vitamin D and bone metabolism. I observe clinically that it is worthwhile to think along with this environment, instead of staring only at a single value. This does not replace conventional medical assessment, but complements it with a view of the connections.
A side effect whose mechanism you know, whose frequency you know and whose risk groups you know is no longer a threat. It becomes a point on the checklist.
Where this topic belongs in the larger context
The phosphate drop is an important detail, but precisely a detail in a larger picture. An iron infusion can bring the most when everything fits together: the right indication, a modern preparation that suits the person, the exclusion of contraindications such as iron overload and good monitoring during and after the dose.
Into this picture the phosphate value fits as an additional aspect that you should know and check when needed. And now you know why some people are recommended a phosphate test after an infusion and others are not. It is not arbitrariness, but an assessment of preparation, person and course.
Frequently asked questions
Can an iron infusion lower the phosphate level?
Yes, some iron preparations can temporarily lower the phosphate level in the blood. The technical term is hypophosphatemia. The hormone FGF23 is responsible, prompting the kidney to excrete more phosphate. How strong the effect is depends clearly on the preparation used. In many people the drop stays mild and causes no symptoms.
How common is hypophosphatemia after an iron infusion?
This depends heavily on the preparation. In a meta-analysis of 42 studies a phosphate drop occurred after ferric carboxymaltose in around 47 percent of those treated, after iron isomaltoside or ferric derisomaltose in only about 4 percent. In two randomized studies the figures for ferric carboxymaltose were even around 75 percent. Most of these drops stay mild and cause no symptoms.
What symptoms can a phosphate deficiency after an iron infusion cause?
Often a mild phosphate drop causes no symptoms at all. If the value falls more markedly and for longer, fatigue, muscle weakness or reduced resilience can appear. These signs resemble those of an iron deficiency and are therefore easily overlooked. In rare, long-lasting cases bone pain can occur.
Why does some iron infusion lower phosphate through FGF23?
FGF23 is a hormone that steers phosphate excretion through the kidney. Some iron preparations temporarily inhibit the breakdown of FGF23, so that more active hormone stays in the blood. The kidney then excretes more phosphate, and the level in the blood can fall. In addition, less active vitamin D is formed, which dampens phosphate uptake in the gut.
Who has a higher risk for a phosphate drop?
A higher risk appears with pronounced iron deficiency and very low ferritin, with normal kidney function, with repeated or high doses and with certain preparations. After bariatric surgery or with chronic inflammatory bowel disease the likelihood is also increased.
How long does the phosphate drop last?
In most people the phosphate level settles again within weeks. In a meta-analysis, however, the value stayed low for several months in a portion of those treated with ferric carboxymaltose. For this reason a check can be useful with repeated doses.
Which iron preparation lowers the phosphate level the least?
In direct comparison studies the phosphate drop occurred much less often with ferric derisomaltose than with ferric carboxymaltose. Which preparation fits in the individual case depends on many factors, however, and belongs in the medical assessment.
Does the phosphate value have to be checked after every iron infusion?
Not with every single dose in the same way. A check can be worthwhile above all when a preparation with higher risk is used, several doses are planned, risk factors are present or symptoms such as lasting weakness appear after the infusion. This is decided individually.
Is a phosphate drop after the iron infusion dangerous?
In the vast majority of cases the drop is mild, temporary and harmless. Severe or long-lasting courses are rare, but can be kept well in view through risk assessment, suitable preparation choice and, where appropriate, a check of the phosphate value.
Is the phosphate drop one of the normal side effects of an iron infusion?
It is a known, preparation-dependent accompanying effect. In the general overview of the side effects of older and modern preparations we place it alongside other reactions. This article looks specifically at the phosphate drop in more detail.
Read on in the iron guide
Sources
- Wolf M, Rubin J, Achebe M, et al. Effects of Iron Isomaltoside vs Ferric Carboxymaltose on Hypophosphatemia in Iron-Deficiency Anemia: Two Randomized Clinical Trials. JAMA. 2020;323(5):432-443. DOI: 10.1001/jama.2019.22450 [RCT, n=245]
- Schaefer B, Tobiasch M, Viveiros A, et al. Hypophosphataemia after treatment of iron deficiency with intravenous ferric carboxymaltose or iron isomaltoside: a systematic review and meta-analysis. Br J Clin Pharmacol. 2020;87(5):2256-2273. DOI: 10.1111/bcp.14643 [Meta-Analysis, 42 studies]
- Coppolino G, Nicotera R, Cernaro V, et al. Iron Infusion and Induced Hypophosphatemia: The Role of Fibroblast Growth Factor-23. Ther Apher Dial. 2019;24(3):258-264. DOI: 10.1111/1744-9987.13435 [Mechanism Review]
- Schoeb M, Räss A, Frei N, et al. High Risk of Hypophosphatemia in Patients with Previous Bariatric Surgery Receiving Ferric Carboxymaltose: A Prospective Cohort Study. Obes Surg. 2020;30(7):2659-2666. DOI: 10.1007/s11695-020-04544-x [Cohort study, n=52]
- Klein K, Asaad S, Econs M, Rubin JE. Severe FGF23-based hypophosphataemic osteomalacia due to ferric carboxymaltose administration. BMJ Case Rep. 2018;2018:bcr-2017-222851. DOI: 10.1136/bcr-2017-222851 [Case Report]
- Teh KK, Chuah MB, Tay SW, et al. Severe Symptomatic Hypophosphataemia as a Complication of Parenteral Iron Replacement. Eur J Case Rep Intern Med. 2020;7(11):001860. DOI: 10.12890/2020_001860 [Case Report]
This article serves general information and does not replace medical advice, diagnosis or treatment. Whether an iron infusion is sensible and safe for you and whether a phosphate check is advisable can only be clarified individually and in a medical conversation.