📚 Part of our Sleep & Stress Guide — an evidence-based guide.
Iron deficiency is the most common nutritional deficiency in the world, and one of the most commonly missed — because the test most people are given cannot detect the stage at which it starts causing symptoms.
The pattern is familiar to a lot of women in particular: exhausted for months, blood work ordered, told it came back normal. In many of those cases the blood work was normal. It was also the wrong test.
Two different conditions
The distinction that resolves most of the confusion is this:
Iron deficiency anaemia means iron stores are gone and haemoglobin has fallen. This is what a complete blood count detects.
Iron deficiency without anaemia means stores are depleted but haemoglobin is still holding in the normal range. A CBC looks entirely normal. Symptoms can be present.
The body defends haemoglobin last. It will drain the ferritin reserve almost completely before red cell production falls, which means a normal haemoglobin does not rule out iron deficiency. By the time anaemia appears, the deficiency has usually been developing for a long time.
The test that finds the earlier stage is ferritin, which reflects stored iron. It has to be asked for. It is not part of a standard CBC.
The number that causes arguments
Here is where honesty is required, because the thresholds are genuinely contested.
Many laboratories flag ferritin as abnormal only below about 15 ng/mL. The World Health Organization uses below 15 µg/L to define depleted stores in adults. But several haematology bodies and guidelines treat values below 30 as consistent with iron deficiency, and this threshold has substantially better sensitivity for detecting genuinely absent bone-marrow iron.
A result of 18 is therefore both “within the reference range” and, on many expert readings, consistent with deficiency. That gap is where a great many symptomatic people fall.
Two caveats matter and both cut against over-interpreting a single value.
First, ferritin is an acute-phase reactant — it rises with inflammation, infection, liver disease and obesity. Someone with an inflammatory condition can have a ferritin of 60 and still be iron deficient. Measuring CRP alongside it helps interpret the number, and transferrin saturation adds information.
Second, the evidence that treating non-anaemic iron deficiency improves fatigue is real but modest. Randomised trials of iron supplementation in non-anaemic women with fatigue have found improvement over placebo — but the effect is moderate, not transformative, and it is clearest in those with the lowest ferritin. Anyone promising that iron will fix your energy is overselling a genuine but limited finding.
Who is actually at risk
Heavy menstrual bleeding is the leading cause in premenopausal women, and it is underreported partly because people have no external reference for what “heavy” means. Pregnancy raises requirements substantially. Blood donation depletes stores faster than most donors realise. Endurance athletes lose iron through several routes at once. Vegetarian and vegan diets provide non-heme iron, which is absorbed less efficiently than the heme iron in meat.
And one group deserves a separate paragraph.
In men and postmenopausal women, iron deficiency is not a nutritional finding until proven otherwise. These groups have no routine route of blood loss, so deficiency raises the question of gastrointestinal bleeding — including from colorectal cancer or a peptic ulcer. Guidelines call for investigation, typically endoscopy, rather than simply prescribing iron. Taking a supplement can correct the number and delay finding the cause. This is the most important thing in the article.
Symptoms, and why they are unhelpful alone
Fatigue, breathlessness on exertion, poor concentration, hair shedding, cold intolerance, brittle nails, headaches. Every one of these has a dozen other causes (the fuller checklist), which is exactly why the symptom list cannot substitute for testing.
Two are more specific and worth knowing. Pica — craving non-food substances, classically ice — is unusual enough to be a genuine clue. And restless legs syndrome has a well-documented association with low brain iron; treating deficiency is part of standard management, and ferritin targets used there are considerably higher than the general threshold.
Correcting it
Diet alone rarely repletes an established deficiency, though it maintains stores. Heme iron from meat, poultry and fish absorbs better than plant iron. Vitamin C substantially improves absorption of non-heme iron; tea, coffee and calcium taken with the meal reduce it.
For supplementation, one finding has changed practice and most people have not heard it. Alternate-day dosing absorbs better than daily dosing. Iron intake triggers a rise in hepcidin, a hormone that blocks further absorption for roughly 24 hours — so a second dose lands in a closed door. Trials have found that giving iron every other day, in a single morning dose, produces greater total absorption than splitting it across each day. It also causes fewer gastrointestinal side effects, which is the main reason people abandon treatment.
Repletion is slow. Haemoglobin responds within weeks; refilling ferritin stores typically takes three to six months of continued treatment after the blood count normalises. Stopping when you feel better is why deficiency so often returns.
Intravenous iron is available for malabsorption, intolerance, or when rapid correction is needed, and is more widely used now than a decade ago.
A word of caution in the other direction: iron is not a benign supplement to take speculatively. Excess is stored, not excreted, and roughly 1 in 200 people of northern European descent carries hereditary haemochromatosis, in which iron accumulation damages the liver, heart and pancreas. Iron is also a leading cause of poisoning deaths in young children. Test before supplementing.
When to see a doctor
Ask for ferritin specifically — not just a CBC — if you have unexplained fatigue, heavy periods, restless legs, or hair loss. Ask for CRP alongside it if you have any inflammatory condition, so the ferritin can be read correctly.
See a doctor before self-treating if you are male or postmenopausal and have low iron, because finding the source matters more than correcting the number.
Seek urgent care for black tarry stools, visible blood in stool, vomiting blood, chest pain or breathlessness at rest, or fainting.
The honest bottom line
Iron deficiency has two stages, and the standard blood test only catches the second one. Ferritin catches the first, but it has to be requested, its reference range is set lower than most experts would use, and inflammation can hide a low value behind a normal-looking number. The treatment effect on fatigue is real and moderate rather than dramatic. And in anyone without menstrual loss, the deficiency itself is the finding that needs explaining — correcting it without asking why is the one mistake worth avoiding entirely.
Frequently asked questions
Can you be iron deficient with normal haemoglobin?
Yes, and it is common. Iron deficiency has two stages: depleted stores with normal haemoglobin, and iron deficiency anaemia, where haemoglobin has also fallen. A complete blood count only detects the second. The body preserves haemoglobin until stores are nearly gone, so symptoms can appear well before anaemia does.
What ferritin level is considered low?
The World Health Organization uses below 15 µg/L to define depleted stores, and many labs flag results only below that. But several haematology guidelines treat values below 30 as consistent with iron deficiency, and that threshold has better sensitivity for genuinely absent marrow iron. A result of 18 can be both inside the reference range and consistent with deficiency.
Should you take iron every day or every other day?
Trials have found greater total absorption with alternate-day dosing in a single morning dose. Iron intake raises hepcidin, a hormone that blocks further absorption for roughly 24 hours, so a same-day second dose is largely wasted. Alternate-day dosing also causes fewer gastrointestinal side effects, which is the usual reason people stop treatment.
Does iron supplementation help fatigue?
In non-anaemic women with low ferritin, randomised trials have found improvement in fatigue over placebo — the effect is real but moderate, and clearest in those with the lowest ferritin. It is not a general energy supplement, and taking iron without testing carries real risk, since excess iron is stored rather than excreted.
How long does it take to correct iron deficiency?
Haemoglobin usually responds within a few weeks, but refilling ferritin stores typically takes three to six months of continued treatment after the blood count has normalised. Stopping as soon as you feel better is a common reason deficiency returns.