A Practical Guide to Iron Deficiency and Anaemia

Why this guide gives you no cut-offs

Almost everything written about iron deficiency is organised around a number. Below this, deficient. Above it, fine. That framing is easy to write and easy to read, and it is the reason so many people are told they are normal while they are still unwell.

There are three problems with it. Reference ranges differ between laboratories and between health systems, so the same blood can be labelled differently depending on who processes it. Those ranges describe the spread of a population that was sampled, not the point at which a person becomes symptomatic. And a single value carries no direction: it cannot tell you whether you are falling, holding or recovering.

So this guide teaches the relationships instead. What each marker represents, how they move against one another, and what a sequence of results over time is telling you. If you want the threshold conversation, my articles on ferritin and the reference range and iron deficiency without anaemia address it directly, and they are the right starting point if you are holding a result right now and want to know what to ask. This guide is for afterwards, when you want to understand the system rather than look up a verdict.

Module 1Iron status as a continuum

Iron balance sits on a spectrum that runs from overload at one end to severe deficiency at the other. Everyone occupies a position on it, and that position moves. Health is not a box you are inside or outside; it is where you sit and which way you are travelling.

This matters because the alternative framing, a fixed line, forces a binary answer onto a continuous quantity. Two people with identical results can have entirely different clinical pictures depending on where each of them was six months ago, what their demands are, and what their own baseline looks like. Evaluating iron status against the individual, rather than against a population cross-section, is the single change that makes the rest of this guide work.

The iron balance equation

Whatever is happening to your iron, it resolves to four mechanisms. Anything that lowers iron status does so through at least one of them, and effective management means identifying which one is operating rather than treating them all the same way.

1. Increased loss. Menstrual blood loss, and particularly heavy menstrual bleeding, is the largest single contributor in premenopausal women. Blood donation counts. So do exercise-related losses through sweat and foot-strike haemolysis.
2. Increased demand. Adolescent growth, pregnancy, and sustained high endurance training loads all raise the amount of iron the body needs to hold its position.
3. Decreased intake. Meat-restricted diets, and low total energy availability, where overall food intake is simply not enough to carry the micronutrients with it.
4. Decreased absorption. Dietary inhibitors such as the tannins in tea and coffee, and calcium taken alongside iron. The absence of enhancers such as vitamin C. And gastrointestinal malabsorption, which is a medical issue in its own right.

The progression

Deficiency does not arrive all at once. It moves through stages, and the marker that most clinical practice screens on is the one that changes last.

Stage 1, depleted stores. Storage iron falls while red cell production carries on unaffected. The body is drawing down its reserve.

Stage 2, iron-deficient erythropoiesis. Transport saturation drops. There is no longer enough iron reaching the bone marrow to make red cells at the usual rate and quality.

Stage 3, overt anaemia. Oxygen-carrying capacity falls past the point the body can compensate for. This is the stage a haemoglobin test is designed to catch.

Iron deficiency without anaemia

Stages 1 and 2 have a name: iron deficiency without anaemia, or IDNA. It describes active tissue-level deficiency, with symptoms, occurring while haemoglobin sits inside its normal reference range.

This is the central problem in how iron deficiency is found and missed. A screening approach built on haemoglobin alone will return a normal result throughout stages 1 and 2. The person is symptomatic, the deficiency is real and treatable, and the test says nothing is wrong. Every month spent in that state is a month of symptoms, and the depletion generally continues while it lasts.

Module 2Symptoms, functional impact and individual risk

Iron is not only about blood. It sits at the centre of oxygen transport, cellular energy production, and several enzyme systems in the brain. That is why the symptom picture is broader than the word anaemia suggests, and why it is so often attributed to something else.

The symptom profile

Iron deficiency can cause symptoms with anaemia, without anaemia, or cause none at all. That single sentence is why it is so often missed: there is no symptom threshold that reliably tracks the deficiency, and being asymptomatic does not mean iron status is fine.

Common symptoms and signs. Fatigue and lethargy. Reduced concentration. Dizziness. Tinnitus. Pallor. Headache.

Restless legs. In susceptible people, iron deficiency promotes restless legs syndrome. It is one of the more specific pointers, and one of the most frequently attributed to something else.

Other presentations. Alopecia. Dry hair or skin. Koilonychia, where the nails become spoon-shaped rather than simply brittle. Atrophic glossitis, a smooth and inflamed tongue.

Pica. The compulsive eating of things that are not food: ice, soil or clay, or raw ingredients such as uncooked rice. It sounds unusual enough that people do not mention it, which is exactly why it is worth asking about.

Beyond symptoms. Iron deficiency worsens the prognosis of heart failure and ischaemic heart disease, and severe iron deficiency anaemia can cause haemodynamic instability. Even when it produces no symptoms at all, iron deficiency can leave function below where it should be.

Symptom profile per Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. Iron deficiency. The Lancet, published online 4 December 2020. doi:10.1016/S0140-6736(20)32594-0

The reason this list matters is that most of it can be explained away. Tiredness gets attributed to work, to sleep, to stress, to age, to parenting. Each of those explanations is plausible on its own, and collectively they are how a treatable deficiency goes unexamined for years.

Who carries elevated risk

• Premenopausal people, and particularly anyone with heavy menstrual bleeding.
• Pregnant and postpartum people, where demand rises sharply and stays raised.
• Endurance athletes and consistently active populations, through the combination of loss and demand.
• Anyone with dietary restriction, or with a known or suspected gastrointestinal absorption problem.

Risk is not diagnosis. Sitting in one of these groups does not mean you are deficient, and sitting outside all of them does not mean you are not. What elevated risk changes is the threshold for investigating, and how seriously an unexplained symptom pattern should be taken.

Module 3The biomarkers, and how they relate to each other

The iron studies

Serum ferritin is the biscuit tin. It is the storage protein, and it tells you what is in reserve. It is the most sensitive early marker of depletion for a simple reason: the body empties the tin before it lets anything else change.

Transferrin is the delivery fleet. It is the protein that carries iron through the blood to the tissues that need it. When iron is scarce the body builds more trucks, trying to capture whatever is available. When iron is plentiful it builds fewer. Transferrin therefore moves in the opposite direction to iron status, which surprises people the first time they see it.

Transferrin saturation, usually written TSAT, is the proportion of those trucks currently carrying a load. It is the closest thing in a standard panel to a measure of how much iron is actually reaching tissue. A large fleet running mostly empty is a specific and informative picture.

Serum iron is the iron circulating at the moment of the draw. It moves through the day, and it responds to a recent meal or supplement, so it carries far less meaning on its own than the other three.

The full blood count

Haemoglobin is the oxygen-carrying protein inside red cells, with iron at its centre. Red cell count and haematocrit describe how many red cells there are and what proportion of blood volume they occupy.

Mean corpuscular volume, or MCV, is the average size of a red cell. Iron deficiency produces smaller cells, so MCV falls. Mean corpuscular haemoglobin and its concentration describe how much haemoglobin each cell carries. Concentration measures are sensitive to plasma volume: dehydration contracts plasma and inflates them, which is one reason the conditions of the blood draw matter.

Red cell distribution width, or RDW, measures how variable red cell size is. It rises in iron deficiency because older normal-sized cells and newly made smaller ones are circulating at the same time. It is often the first index to move.

Platelets can rise in iron deficiency. On their own they mean little; alongside the rest of the picture they are another piece of corroboration.

Reading the markers against one another

No single marker settles the question. The information is in the combinations, because different conditions produce different patterns across the panel.

That third row is the one to sit with. A raised ferritin with an inflammatory marker alongside it can look reassuring on a report and mean the opposite. Reading ferritin without CRP is how functional iron deficiency gets missed, and it is the single most common reason a result is interpreted backwards.

Your own trajectory

The most useful comparison you have is not against the population. It is against yourself, six months ago.

For that comparison to mean anything, the conditions have to be consistent: a morning draw, fasted, rested beforehand, and where relevant at a comparable point in the menstrual cycle. Iron markers move with all of these, and a result taken under different conditions is not comparable to the one before it. Two results collected carelessly can differ by more than a real change would.

Read the direction against what has changed in your life. A shift in contraception, a rise in training load, a change in diet, a pregnancy, a period of heavier bleeding: these are the things that move a trajectory, and pairing the trend with the cause is what turns a set of numbers into a decision.

Module 4Repletion, matched to the mechanism

There is no single correct way to restore iron. What works depends on which of the four mechanisms is operating, how far depletion has progressed, and what a person can actually tolerate and sustain.

Treatment that ignores the mechanism fails predictably. Supplementing someone whose losses exceed what they can absorb will not hold. Adding iron to a diet when the problem is malabsorption does not address the problem. Recommending dietary change that a person will not maintain is not a plan.

That last point deserves saying plainly. Tailoring repletion to a vegan or vegetarian is a technical problem with technical answers, not an invitation to suggest they eat meat. Advice a person will not follow has no effect, however sound it looks on paper.

Diet and absorption

Iron from animal sources, haem iron, is absorbed more readily than the non-haem iron in plants. That is a difference in efficiency rather than a verdict on either diet, and non-haem absorption is strongly modifiable.

Enhancers. Vitamin C taken with an iron source meaningfully improves non-haem absorption. Pairing rather than separating is the practical move.

Inhibitors. Tannins in tea and coffee, calcium, and phytates all reduce absorption. The realistic response is timing rather than elimination: separating them from iron-containing meals or supplements captures most of the benefit without asking anyone to give up their morning coffee.

Oral iron

Formulations differ. Ferrous salts are the long-standing option and the cheapest. Iron polymaltose, and the liposomal and sucrosomial preparations, are generally better tolerated, at higher cost and with different absorption behaviour.

The dosing rhythm matters more than most people are told. Taking iron raises hepcidin, the hormone that regulates absorption, and that rise blunts uptake for a period afterwards. Dosing on alternate days rather than daily works with that rhythm instead of against it. The practical result is better absorption per dose and, for many people, considerably fewer gastrointestinal side effects.

Tolerability is not a side issue. The most effective supplement is the one that gets taken for the whole course, and gastrointestinal upset is the main reason courses are abandoned before they have done anything.

Intravenous iron

Intravenous iron is indicated in specific situations rather than as a default: genuine intolerance of oral iron, malabsorption, ongoing blood loss that outpaces oral repletion, or a clinical need to correct quickly. Modern formulations, such as ferric carboxymaltose and ferric derisomaltose, allow substantial replacement in few visits.

What is least well explained to patients is the timeline. Blood markers can respond quickly, and the redistribution of iron into muscle and other tissues takes roughly four to eight weeks. Someone whose ferritin has corrected may still be waiting for how they feel to catch up. Knowing that in advance is the difference between a normal recovery and the conclusion that the treatment did not work.

Module 5Long-term monitoring

Correcting iron once is the straightforward part. Holding the correction is where most of the value is, and where most plans quietly stop.

Retesting

A reasonable pattern is to retest roughly six to twelve weeks after treatment, and then every three to six months while stability is established, with a view over one to two years rather than a single follow-up. The purpose is to see whether ferritin has stabilised and transferrin saturation has recovered, not simply whether a symptom eased.

Symptoms alone are an unreliable endpoint. They fluctuate for many reasons, and a short-term dip in fatigue can be mistaken for repletion when stores are still low. The trajectory is the evidence.

Resolving the cause

If the mechanism driving loss is still operating, the deficiency will return. Repletion buys time; it does not settle the question.

That means heavy menstrual bleeding needs to be evaluated as the clinical issue it is rather than accepted as normal, with gynaecological input where it is warranted. It means unexplained loss in anyone not menstruating warrants gastrointestinal investigation. And it means the plan has to survive the changes ahead of it: pregnancy, a rise in training, a dietary shift, perimenopause. Each of those moves the balance, and a trajectory that was stable under one set of demands will not necessarily hold under the next.

The shape of it

The pattern this guide describes is one loop, repeated. Identify which mechanism is operating. Match the repletion to it. Confirm the recovery with a trajectory rather than a single value. Address the cause so the correction holds. Then watch it across the life stages that will test it.

None of that requires you to memorise a cut-off. It requires you to know what the markers represent, to see them as a system rather than a list, and to hold a record of your own over time. That is a more demanding way to read a blood test, and it is the one that will not tell you that you are fine when you are not.