Your blood test looks fine. So why are you still feeling exhausted, or finding that your usual workout takes more effort than it used to? Sometimes, the answer lies in what a routine blood test doesn't tell you about your body's iron reserves.
Haemoglobin, the protein in red blood cells that carries oxygen, is routinely measured in a full blood count. However, normal haemoglobin does not necessarily mean iron stores are healthy. That's where ferritin comes in. This protein stores iron, and its levels can fall before haemoglobin drops enough to indicate anaemia.
The tricky part is that low iron stores don't always announce themselves. Some people feel perfectly well, eat a balanced diet and exercise regularly, yet have depleted reserves. Others notice fatigue, reduced exercise tolerance or difficulty concentrating, changes that can easily be blamed on poor sleep, stress or a demanding routine.
So, what does low ferritin actually mean, why might iron stores remain low despite supplements, and when should a persistent deficiency prompt further investigation? The answers involve more than a single blood test or a higher dose of iron.
Two measures, two different stories
Iron is needed to make haemoglobin, but it also supports energy production, muscle function and normal cognitive function. Ferritin is a marker of the body's stored iron, while haemoglobin reflects the blood's capacity to carry oxygen.
Dr Yousef Said, Medical Director at Metabolic, says a common misconception is that normal haemoglobin levels mean iron stores are adequate. "Haemoglobin is a late marker. Iron stores can become low long before this affects the red blood cells. At first, the body uses its remaining iron reserves to keep hemoglobin at a normal level. If those reserves continue to fall, hemoglobin can eventually drop as well. Ferritin shows you the reserves before you reach that point.”
Dr Azeem Irshad, Specialist Internal Medicine at Aster Clinic, Al Nahda, Dubai, makes the same distinction: "The iron deficiency can occur without anaemia. This is important because the body generally uses up its stored iron before haemoglobin necessarily falls.”
Ferritin is therefore best interpreted alongside haemoglobin, the full blood count and other relevant iron markers, rather than in isolation.
What counts as low?
So, what ferritin level should raise a question? The answer depends partly on the person and their health, but the numbers offer a starting point.
Ferritin is usually measured in nanograms per millilitre (ng/mL), although reference ranges vary between laboratories. In adults, a level below 30 ng/mL generally indicates depleted iron stores, while a reading below 15 ng/mL is strong evidence of iron deficiency, explains Dr Said.
However, the number alone cannot explain what's happening. Why have iron stores fallen? Is the body losing iron faster than it can replace it, or is it struggling to absorb enough? Symptoms, haemoglobin levels, other iron markers and medical history all help doctors work out what a result means and what might be behind it.
There is another wrinkle: Ferritin can rise in response to inflammation. That means a result within the laboratory's normal range, or even a high reading, may not tell the whole story.
As Dr Azeeza Safwan Amoori, Specialist Internal Medicine at Medcare Medical Centre, cautions: “A normal serum ferritin doesn't necessarily exclude iron deficiency particularly in the presence of inflammation or chronic disease.”
When the numbers don't tell a clear story, doctors can look at other markers. Transferrin saturation helps show how much circulating iron is available for the body to use, while serum iron and total iron-binding capacity provide additional information about iron status. C-reactive protein (CRP), a marker of inflammation, can help doctors assess whether inflammation is affecting the ferritin result.
Subtle symptoms, or none at all
You might find yourself flagging halfway through a workout you usually manage comfortably. Concentrating at work might take more effort, or you may feel unusually drained despite getting a decent night's sleep. Or you might notice absolutely nothing.
Low ferritin does not follow a single pattern. “A person can therefore have depleted iron reserves while the haemoglobin remains within the normal range. Some people may experience fatigue, reduced exercise tolerance, difficulty concentrating or restless legs, while others may have no symptoms at all,” explains Dr Azeeza.
A part of the difficulty is that these changes rarely point neatly to one cause. Feeling exhausted? It could be poor sleep or a demanding week. Struggling to concentrate? Stress might seem the obvious explanation. Even a dip in exercise stamina can be easy to dismiss as being out of practice. When changes creep in gradually, it is not always obvious that something has shifted.
But symptoms are clues, not a diagnosis. Fatigue and poor concentration can be associated with thyroid disorders, other nutritional deficiencies and numerous medical conditions. Hair shedding has many possible causes, too, while restless legs may need a separate assessment. Persistent symptoms or abnormal blood results deserve attention, but neither automatically means iron deficiency is to blame.
If iron deficiency progresses to anaemia, the effects may become more noticeable. Breathlessness during exertion, dizziness, headaches, palpitations, weakness and pallor can occur as haemoglobin levels fall. Such symptoms warrant medical assessment to establish their cause, rather than simply assuming low iron is responsible.
The catch, is that there may be no obvious warning sign at all. Someone can feel perfectly well and still have depleted iron stores. That is why symptoms alone cannot tell the whole story; blood results, interpreted alongside medical history and other relevant tests, matter too.
When supplements don’t seem to work
Eating iron-rich foods or taking supplements does not always mean the body's reserves will recover as expected. If ferritin remains low, the answer may not be to take more iron. The first step is to work out whether the body is getting enough, absorbing it effectively and holding on to what it receives.
Dr Irshad says possible explanations include inadequate iron intake, reduced absorption, increased requirements, ongoing blood loss or insufficient replacement.
Sometimes, the issue is how iron is absorbed. Tea and coffee consumed around the time of an iron-rich meal or supplement can reduce iron absorption, while calcium can interfere in certain circumstances. Some medications may also play a role. Proton-pump inhibitors, which reduce stomach acid, can contribute to impaired iron absorption in some patients.
In other cases, the body may be losing iron faster than it can replace it. Heavy menstrual bleeding and frequent blood donation can deplete iron stores. Blood loss from the digestive tract is another possibility, sometimes associated with conditions such as stomach ulcers, intestinal polyps or inflammatory bowel disease.
Less common, gastrointestinal bleeding may be linked to bowel cancer, which is one reason persistent unexplained iron deficiency warrants medical investigation.
There is also the question of whether the gut can absorb enough iron in the first place. Coeliac disease, Helicobacter pylori infection and autoimmune atrophic gastritis can interfere with iron absorption. Inflammatory bowel disease and previous gastric or bariatric surgery may also affect how well the body takes up nutrients.
As Dr Irshad explains, “Therefore, when ferritin repeatedly remains low despite appropriate replacement, the important question becomes not simply ‘How much iron should we give?’, but ‘Why are the iron stores not being replenished?’”
The dose and timing of supplementation also deserve review. Taking more iron is not automatically the solution, and treatment should be guided by a clinician, with follow-up testing to establish whether iron stores are recovering.
When low ferritin points to something more
Sometimes, the reason for low ferritin is relatively straightforward. In other cases, the result raises questions that cannot be answered by diet, supplements or menstrual history alone.
The challenge is that some causes of iron deficiency produce few obvious clues. Blood loss from the digestive tract, for instance, may happen without visible blood or noticeable digestive symptoms. Coeliac disease can also interfere with iron absorption, sometimes even in people who do not experience the familiar digestive complaints.
“Low ferritin is a signal pointing to something, not a diagnosis in itself,” says Dr Said.
Finding the explanation starts with the person's wider medical picture. A clinician may consider their age, sex, symptoms, medical history and previous blood results before deciding whether further investigation is appropriate. Depending on the circumstances, this could include screening for coeliac disease, testing for Helicobacter pylori or investigating possible gastrointestinal blood loss.
The decision can be particularly important for men and postmenopausal women, in whom menstrual blood loss cannot explain depleted iron stores. Other warning signs include deficiency that persists despite appropriate treatment or results that suggest continued iron loss.
This does not mean low ferritin automatically signals a serious illness. It means doctors need to consider the full picture rather than settling on an explanation that does not account for the results.
Why the trend matters
A ferritin result tells you where iron stores stand at one point in time. Repeat testing adds another piece of the puzzle: Are those reserves rebuilding, staying depleted or running down again?
The difference matters. A steady rise after treatment suggests iron stores are recovering. But if ferritin climbs during treatment only to fall again afterwards, doctors may need to consider whether iron is still being lost, absorption remains impaired or the underlying cause has not been resolved.
Ferritin is not interpreted in isolation. Doctors may compare it with haemoglobin, red-cell indices and transferrin saturation, taking inflammatory markers into account when appropriate. Symptoms, medical history, diet and how the person has responded to treatment all help put the numbers into context.
“The important point is not simply whether the number has moved up or down, but what that pattern means in the context of the individual,” explains Dr Azeeza.
A result that remains low does not automatically point to a serious problem, just as a rise does not necessarily mean the underlying issue has been resolved. The next step depends on the extent of the deficiency, the pattern over time and the person's response to treatment. Sometimes monitoring is enough; in other cases, the trend provides a reason to investigate further.
When treatment falls short
What happens when ferritin refuses to budge, even after weeks of taking iron? The answer is not necessarily a higher dose.
Dr Said says ferritin is often rechecked eight to 12 weeks after treatment begins, depending on the individual. If levels have not meaningfully improved after around three months of consistent, correctly taken treatment, the plan may need to be reviewed.
That could involve checking the dose, timing and consistency of supplementation, assessing whether the body is absorbing iron properly, or investigating continued blood loss. For some people, intravenous iron may be an option if oral treatment is ineffective or unsuitable.
Taking high-dose iron indefinitely without medical guidance is not the answer. Supplements can cause side effects, and unnecessary iron can be harmful. More importantly, repeatedly treating the deficiency without investigating why it developed may leave the underlying problem unresolved.
As Dr Said advises: “Iron is something to measure and understand, not guess.”
The goal is not simply to push ferritin into the reference range. It is to replenish iron stores, address the reason they became depleted and establish that the improvement is sustained.
Ferritin can reveal a problem even when haemoglobin looks reassuring. But its real value lies in the bigger picture: how the result fits with other blood tests, the person's circumstances and the way their iron stores change over time.
In a 2017 study published in The Lancet Haematology, research offers one reason why taking more iron more frequently is not always better. In the study, in Switzerland, researchers found that iron-depleted women absorbed a higher proportion of their iron when they took it every other day rather than daily: 21.8 per cent compared with 16.3 per cent.
The researchers linked this difference to hepcidin, a hormone that rises after an iron dose and temporarily reduces the body's ability to absorb more iron. However, the study measured iron absorption in a small group of women, so the findings do not mean alternate-day dosing is best for everyone. The right schedule depends on the individual and should be discussed with a doctor.
So, if your ferritin is low, don't focus on the number alone. Ask your doctor what the result means alongside your haemoglobin and other iron markers, what might have caused your iron stores to fall, and when your blood tests should be repeated.
If you're already taking iron but your levels are not improving, or they fall again after treatment, arrange a review rather than increasing the dose yourself. Your doctor may need to reassess how you're taking the supplement, check for problems with absorption or ongoing blood loss, and decide whether further tests or a different treatment are needed.
The aim is not simply to get ferritin back into range. It is to replenish your iron stores, address the reason they became depleted and check that the improvement lasts.