Functional iron deficiency – The hidden deficiency affecting your energy

Functional iron deficiency (FID) is a condition in which the body has iron, but cannot use it effectively where it is needed most, especially in the bone marrow, muscles and other tissues. The result can be symptoms that affect everyday life, exercise, recovery and work capacity, even before clear anemia develops. Current guidelines and reviews describe that ferritin and transferrin saturation need to be interpreted together, especially in inflammation, heart failure, kidney disease and other chronic conditions.

What does functional iron deficiency mean?

Functional iron deficiency means that the body does not have enough available iron to meet its needs, even though iron may be stored. Simply put: the iron is “in storage”, but does not get to the right place at the right time.

This is different from absolute iron deficiency, where the body’s iron stores are actually depleted. In functional iron deficiency, ferritin – a blood test that reflects iron stores – can be normal or even elevated, while transferrin saturation (TSAT) is low. This indicates that too little iron is circulating and biologically available. In newer renal medicine guidelines, the term iron-limited erythropoiesis is increasingly used instead of functional iron deficiency, as it better describes what is happening: the bone marrow receives too little usable iron to form red blood cells.

An important explanation is the hormone hepcidin. Hepcidin regulates how much iron is absorbed from the intestine and how much is released from the body's stores. In inflammation, hepcidin rises, which causes iron to be "locked up" in the body's stores. In this case, you can have symptoms and sometimes anemia even though ferritin does not appear low. WHO also emphasizes that ferritin rises during inflammation and can therefore underestimate iron deficiency if interpreted alone.

What symptoms can functional iron deficiency cause?

The symptoms are often non-specific and can therefore easily be explained away as stress, lack of sleep or high workload. But iron is not only needed for hemoglobin, but also for energy metabolism, muscle function and the brain's signaling system.

Common symptoms of functional iron deficiency include:

• fatigue or unusual exhaustion
• decreased physical performance
• shortness of breath on exertion
• palpitations
• difficulty concentrating or “brain fog”
• headaches
• restless legs
• poor recovery after exercise
• feeling cold or generally weak

It is important to understand that you can have these symptoms even without obvious anemia. Population data from the United States show that both absolute and functional iron deficiency occur even in people without anemia, and the condition is especially relevant in cases of concomitant inflammation or obesity.

In practice, it can look like this:

• The parent of a small child who does not recover despite sleeping when given the opportunity.
• The exerciser who suddenly loses stamina despite unchanged training.
• The person with an inflammatory disease who has “normal ferritin”, but still has low energy.
• The employee in a workplace who functions, but with reduced energy, concentration and productivity for a long time.

For employers, this is relevant because low iron availability can contribute to reduced performance, increased fatigue and longer recovery, even before the condition has had time to become a clear anemia.

Why does functional iron deficiency occur?

The most common mechanism is inflammation. When the immune system is activated, hepcidin increases, which reduces both intestinal iron absorption and release of stored iron. This is seen in, among other things:

• chronic kidney disease
• heart failure
• inflammatory bowel disease
• autoimmune diseases
• infections
• cancer
• overweight/obesity with low-grade inflammation

In heart failure, iron deficiency is very common, and guidelines often use the combination of ferritin < 100 µg/L, or ferritin 100–299 µg/L together with TSAT < 20%, to identify treatable iron deficiency. At the same time, more recent analyses show that TSAT and serum iron often reflect clinically relevant iron deficiency better than ferritin alone.

In chronic kidney disease, the problem is similar. There, functional iron deficiency or iron-limited erythropoiesis is well known, especially in people with ongoing inflammation or treatment that stimulates blood formation. New KDIGO guidelines describe low TSAT despite normal or high ferritin as a central pattern.

In gastrointestinal diseases, especially inflammatory bowel disease, the picture is further complicated by both inflammation, poor absorption and sometimes blood loss. Therefore, ferritin up to about 100 µg/L can still be compatible with iron deficiency when inflammation is present at the same time.

How is the diagnosis made?

It is not possible to determine functional iron deficiency with a single test. The assessment is based on a comprehensive picture of blood status, iron tests and sometimes inflammation markers.

Common tests are:

• Hb (hemoglobin) – shows whether anemia is present
• Ferritin – reflects iron stores, but increases with inflammation
• Serum iron – varies during the day and should be interpreted with caution
• Transferrin – shows the body's iron transport capacity
• Transferrin – helps assess whether inflammation can affect ferritin
• sometimes also Transferrinreceptor or reticulocyte parameters in more complex case

A common patient question is: “Can I have iron deficiency even if ferritin is normal?”
Yes, you can. Especially if there is inflammation, heart failure, kidney disease or another chronic disease at the same time. In this case, ferritin can look “fine” even though available iron is too low. Low TSAT, often below 20%, then raises suspicion of functional iron deficiency.

Another common question is: “Do I have to have low Hb for it to be important?”
No. Iron deficiency can cause symptoms before the hemoglobin drops. That is precisely why it is valuable to check several iron markers and not just the blood count.

If iron deficiency is confirmed, you also need to consider the cause. Heavy bleeding is common in menstruating women. In others, the cause may be diet, blood donation, gastrointestinal bleeding, inflammation, impaired absorption, or combinations of several factors. In adult men and postmenopausal women, newly discovered iron deficiency or iron deficiency anemia often requires special investigation of a possible source of bleeding from the gastrointestinal tract.

Treatment and what you can do yourself

Treatment depends on why the functional iron deficiency has arisen and how pronounced the symptoms are.

Common principles are:

• treat the root cause, for example inflammation or bleeding
• replenish iron if deficiency is detected
• follow up with new tests to see if the treatment is working
• choose a form of treatment based on diagnosis, tolerance and disease picture

Oral iron works well for many with classic iron deficiency, but in functional iron deficiency the effect may be worse if hepcidin is elevated and intestinal absorption is inhibited. This is particularly relevant in active inflammation. In certain conditions, such as heart failure and sometimes chronic kidney disease, intravenous iron is often used instead when the treating physician deems it appropriate. In heart failure, major cardiology guidelines recommend intravenous iron in selected patients with iron deficiency to improve function and quality of life, although later studies have nuanced the effect on severe outcomes such as hospitalizations and death.

What you can do yourself is, among other things:

• seek assessment in case of persistent fatigue, low energy or recurring restless legs
• do not settle for just “normal blood counts” if symptoms persist
• think about an iron-rich diet, such as meat, legumes, whole grains and green leafy vegetables
• be aware that absorption can be affected by concomitant inflammation, stomach disease or certain medications
• follow the prescription carefully if iron therapy is started

In everyday life, it is also wise to see iron status as part of the whole. Of course, fatigue can be caused by many other things, such as thyroid disorders, B12 or folate deficiencies, sleep problems, or stress. Therefore, targeted testing is often the safest way to get clear answers.

When should you get tested?

It is especially wise to check your iron status if you:

• are tired for a long time without a clear explanation
• have reduced fitness or recovery
• have heavy periods
• are pregnant or have recently been pregnant
• are a blood donor
• eat a very monotonous diet or a vegetarian/vegan diet without planning
• have a chronic inflammatory disease
• have heart failure or kidney disease
• have gastrointestinal symptoms or suspected impaired absorption
• have a normal Hb but still have symptoms consistent with iron deficiency

If you want to get a fair picture, a single test is rarely enough. A combination of blood count, ferritin, transferrin saturation and sometimes inflammation markers is often needed. This makes the interpretation more reliable and reduces the risk of functional iron deficiency being overlooked.