Iron deficiency: why your blood test looks normal

Haemoglobin and ferritin: two measures, two questions

Haemoglobin measures the present

Haemoglobin is the protein in red blood cells that carries oxygen. When it falls below a threshold, this is called anaemia. It is an indicator of consequence: it says the shortage of iron has become large enough to affect the production of red blood cells. As long as it is normal, the body is still managing that job.

Ferritin measures the stores

Ferritin is the protein that stores iron. Measuring it reflects the state of the reserve, and it is the one that falls first, often well before haemoglobin. That is why someone can have a perfectly normal blood count and near-empty stores.

The practical consequence

If only haemoglobin was measured, a “normal” result answers only one question out of two. This is precisely the point worth raising in consultation, especially with heavy periods, closely spaced pregnancies, a vegetarian diet or regular sport.

Being short of iron without being anaemic

A common and real situation

Iron deficiency without anaemia is described as very widespread among women who are physically active^[1]. It long stayed in a blind spot: since haemoglobin is normal, the picture fits no classic box, and the fatigue is put down to a busy life.

The trial that tested the question

Twenty-six active women, iron-deficient but not anaemic, were randomly allocated to intravenous iron or a placebo, double-blind. Performance, fatigue, mood and quality of life were assessed before the intervention, then four days and four weeks afterwards^[1].

The results

Iron status improved, with a highly variable individual response. No difference appeared in maximal oxygen uptake, lactate threshold or haemoglobin mass. Running economy did improve, however, and above all fatigue scores improved in the iron group but not on placebo, after four weeks^[1]. Mood and quality of life did not change.

The two clocks: why the fatigue persists

What corrects quickly

Once treatment starts, red blood cell production picks up quickly and haemoglobin rises within a few weeks. This is the value checked first, and it is the one that comes back “normal” at the follow-up visit.

What takes months

Refilling the stores is another matter entirely. The reserve rebuilds slowly, which is why treatment is usually continued for several months after haemoglobin has normalised. Stopping as soon as the blood count looks good is the most common mistake: it leaves the stores empty and sets up the next relapse.

The gap as it is lived

This gap explains the sentence “my results are normal but I am still tired”. Two different questions then arise with the doctor: was ferritin measured, and was treatment continued long enough to rebuild the reserve? The exact duration depends on how deep the deficiency is and on how well the treatment is tolerated.

The pitfall: a falsely reassuring ferritin

Why it can mislead

Ferritin is not only a marker of the stores: it is also an acute-phase protein, whose level rises with inflammation, infection or chronic disease. Someone who genuinely lacks iron can therefore show a ferritin within the reference range, or even a raised one, if an inflammatory process is under way.

What this implies

This is why a ferritin result is interpreted in context, often alongside an inflammatory marker such as C-reactive protein. A normal ferritin together with inflammation does not rule out a deficiency, and further iron studies may then be needed.

The right question to ask

Rather than “is my ferritin normal”, the useful question becomes: “is my ferritin interpretable in my situation?” This nuance is rarely explained in consultation for lack of time, and yet it accounts for a share of the cases where symptoms persist despite results judged satisfactory.

Absorbing iron better day to day

Two forms, two rates of absorption

Iron from animal products, known as haem iron, is better absorbed than iron from plants. The latter is not negligible for all that: lentils, chickpeas, tofu, pumpkin seeds and green vegetables all provide iron, as long as the conditions for absorption are looked after.

What helps and what hinders

Vitamin C clearly improves the absorption of plant iron: citrus, peppers or parsley in the same meal is enough. Conversely, tea and coffee taken during the meal reduce it sharply, as does calcium in high doses. Shifting the hot drink by an hour is a simple and often overlooked step.

The dosing rhythm: what Zurich showed

Here is a finding that upsets habit, and it comes from Switzerland. Two randomised trials at the Swiss Federal Institute of Technology in Zurich, in women aged 18 to 40 with low stores, compared dosing schedules. The result: giving iron every other day allowed more of it to be absorbed than a daily dose, with cumulative absorption of around 22 % versus 16 %, and 175 mg of iron actually absorbed versus 131 mg^[2].

Why less often absorbs better

The explanation lies with a hormone, hepcidin, which blocks iron absorption and rises after each dose. Spacing the doses gives it time to come back down. The same work also compared a single morning dose with two half-doses spread through the day: splitting did not improve absorption and raised hepcidin^[2]. In other words, the classic advice to spread doses through the day is not the best option.

These findings concerned women who were deficient but not anaemic, and the authors called for confirmation in anaemic patients. So it is a question to put to your doctor rather than to apply on your own, all the more so as the schedule also depends on digestive tolerance.

What diet does not do

It maintains iron status; it does not correct an established deficiency. When the stores are low, no dietary rebalancing replaces the treatment decided by a doctor. Confusing the two costs months, often at the price of fatigue that settles in.

Look for the cause, not just the number

A deficiency always has an origin

Correcting without understanding is like filling a leaking bucket. The most common causes in women of childbearing age are heavy periods, closely spaced pregnancies and breastfeeding. Then come digestive losses, absorption disorders including coeliac disease, and insufficient intake.

When finding the cause becomes a priority

In a man, in a woman after the menopause, or where deficiency returns despite well-conducted treatment, the origin must be actively sought. This is not a formality: sometimes that is exactly where the real diagnosis lies, and it is entirely a matter for the doctor.

Frequently asked questions