Iron Deficiency: Why It Happens and When It Matters

Understanding low iron, how it is treated, and why finding the underlying cause matters

Iron deficiency is common.

Sometimes it causes obvious symptoms such as tiredness, shortness of breath or feeling faint. Sometimes it is found on a routine blood test before anaemia develops.

The important point is that low iron is not a diagnosis by itself.

It is a finding that needs two questions answered:

How low are your iron stores — and why are they low?

Replacing the iron matters. Finding the cause can matter even more.

What does iron actually do?

Iron is needed to make haemoglobin, the protein in red blood cells that carries oxygen around your body.

Your body also uses iron in muscles, enzymes and many other processes.

When your iron stores begin to run down, you develop iron deficiency.

If the deficiency progresses far enough that you can no longer make enough haemoglobin, you develop iron-deficiency anaemia.

These are related, but they are not the same thing.

You can therefore be iron deficient even when your haemoglobin is still normal.

What is ferritin?

Ferritin is a protein that stores iron.

A ferritin blood test is therefore one of the most useful ways to estimate how much stored iron you have available.

In adults, a ferritin below about 30 micrograms per litre (µg/L) is commonly used as evidence of iron deficiency.

But ferritin has an important limitation.

• inflammation
• infection
• liver disease
• some cancers
• other illnesses

This means a low ferritin is very helpful because it confirms iron deficiency, but a ferritin in the “normal” range does not always rule it out if inflammation or illness is present.

This is why I do not interpret ferritin in isolation.

What are iron studies?

• ferritin
• serum iron
• transferrin
• transferrin saturation

Patients sometimes focus on the serum iron result because it looks like the obvious number to follow.

In practice, serum iron can fluctuate and is usually less useful on its own than ferritin and the broader clinical picture.

A full blood count is also important because it tells us whether iron deficiency has progressed to anaemia and gives information about the size of the red blood cells.

What symptoms can low iron cause?

Iron deficiency may cause no symptoms at all.

As it becomes more significant, people may notice:

• tiredness
• reduced exercise tolerance
• shortness of breath
• weakness
• dizziness or feeling faint
• headaches
• palpitations
• difficulty concentrating

Symptoms are not specific to iron deficiency, though.

Fatigue, for example, has many possible causes. A borderline ferritin should not automatically become the explanation for every symptom.

The blood results and the person both matter.

Why might my iron be low?

There are four broad possibilities:

1. You are losing iron
2. You are not taking in enough iron
3. You are not absorbing it properly
4. Your body’s iron requirements have increased

The likely cause varies substantially with age, sex, medical history and circumstances.

Heavy periods are a common cause

For menstruating women, heavy menstrual bleeding is one of the most common reasons for iron deficiency.

If you regularly:

• soak through pads or tampons quickly
• need double protection
• pass large clots
• bleed for prolonged periods
• need to change protection overnight
• arrange your life around your periods

it is worth discussing this with your GP.

Replacing iron may make you feel better, but if heavy bleeding continues, the deficiency may simply return.

The useful question is therefore not only: “How do we replace the iron?” but: “Can we reduce the ongoing blood loss?”

What about bleeding from the bowel or stomach?

Blood loss from the gastrointestinal tract is another important cause.

Sometimes it is obvious, but often it is not visible.

Possible causes include:

• stomach or duodenal ulcers
• inflammation in the bowel
• polyps
• bowel cancer
• haemorrhoids
• medicines that increase bleeding risk
• other gastrointestinal conditions

This does not mean that everyone with low iron has bowel cancer.

But unexplained iron deficiency deserves more careful investigation in people where menstrual blood loss is not an obvious explanation — particularly men and postmenopausal women.

Replacing iron without understanding the cause may allow the underlying problem to continue.

Can diet cause iron deficiency?

Yes, but diet is only one possible explanation.

Iron-rich foods include:

• red meat
• poultry
• fish and seafood
• legumes
• tofu
• iron-fortified cereals
• some nuts and seeds
• green leafy vegetables

Iron from animal foods is generally absorbed more easily than iron from plant foods.

Vegetarians and vegans can still obtain enough iron, but they may need greater attention to iron-rich foods because plant-based iron is absorbed less efficiently.

Can poor absorption cause it?

Yes.

You can eat adequate iron and still become deficient if your gut cannot absorb enough.

Conditions that can contribute include:

• coeliac disease
• inflammatory bowel disease
• previous stomach or bowel surgery
• some chronic gastrointestinal conditions

Certain medicines may also reduce absorption.

If iron deficiency repeatedly returns despite appropriate treatment, poor absorption is one possibility worth considering.

What about pregnancy or blood donation?

Pregnancy increases iron requirements because iron is needed for both the mother’s expanding blood supply and the developing baby.

If you are pregnant, iron results and treatment should be interpreted in that context.

Regular blood donation can also gradually lower iron stores because each whole-blood donation removes iron from the body.

If you donate frequently and your ferritin is low, mention your donation history to your GP.

Do I need iron tablets?

Often, yes.

Oral iron is usually the first treatment for uncomplicated iron deficiency.

Different products contain different amounts of elemental iron, which is the amount of iron actually available in the dose.

The best dose and schedule depends on how deficient you are, whether you are anaemic, your symptoms and how well you tolerate treatment.

Lower doses or alternate-day dosing may sometimes be easier to tolerate and still effective.

Your GP or pharmacist can help choose a preparation and schedule that suits you.

How should I take oral iron?

• constipation
• nausea
• abdominal discomfort
• diarrhoea
• dark stools

Dark stools are expected and are not harmful by themselves.

Tea, coffee, calcium and some medicines can reduce iron absorption when taken at the same time, so timing can matter.

Vitamin C may improve absorption, but you do not usually need large doses.

If one preparation causes significant side effects, do not assume that oral iron will never work for you. A different product, lower dose or different schedule may be better tolerated.

Treatment often continues for a period after haemoglobin improves so that iron stores can recover.

A repeat blood test is usually used to confirm that treatment is working.

When would I need an iron infusion?

An intravenous iron infusion delivers iron directly into a vein.

It can replenish iron more quickly than tablets, but that does not mean it is automatically better.

IV iron may be considered when:

• oral iron is not tolerated
• oral iron has not worked adequately
• absorption is poor
• iron needs to be replaced more quickly
• there is significant ongoing blood loss
• another medical condition makes IV replacement more appropriate

Possible side effects include headache, nausea, temporary flu-like symptoms and, rarely, a serious allergic reaction.

Iron can also leak into the tissues around the infusion site and cause long-lasting skin staining, which is uncommon but important to know about.

An infusion treats the deficiency.

It does not explain why the deficiency happened.

That investigation still matters.

Is an iron infusion a quick fix for tiredness?

Not necessarily.

An iron infusion can be very effective when someone genuinely has iron deficiency.

But fatigue alone is not a reason to receive IV iron.

If your iron stores are adequate, giving more iron is unlikely to fix an unrelated cause of tiredness and exposes you to unnecessary treatment.

More treatment is not automatically better treatment.

Can you take too much iron?

Yes.

Iron is not harmless.

Do not take high-dose iron indefinitely without confirming that you are actually deficient.

Too much iron can accumulate and damage organs.

A high ferritin does not automatically mean iron overload, because ferritin can also rise with inflammation, liver disease and other conditions.

When iron overload is suspected, ferritin is interpreted alongside other tests and the broader clinical picture.

What should my GP investigate?

This depends heavily on the patient.

I may consider:

• the degree of iron deficiency
• whether anaemia is present
• periods and other blood loss
• pregnancy
• diet
• blood donation
• bowel or stomach symptoms
• medicines
• coeliac disease or poor absorption
• age and cancer-screening status

The same ferritin result can therefore lead to very different plans in different people.

A 25-year-old woman with clearly heavy periods is not the same clinical situation as a 65-year-old man with newly diagnosed iron deficiency.

This is why the cause matters as much as the number.

When should I seek medical review sooner?

Arrange prompt medical assessment if iron deficiency is accompanied by:

• black, tar-like stools that are not simply from iron tablets
• visible blood in the stool
• vomiting blood
• significant unexplained weight loss
• difficulty swallowing
• persistent abdominal pain
• fainting
• significant shortness of breath
• chest pain
• very heavy or uncontrolled bleeding

These symptoms do not tell us what the cause is, but they should not simply be managed with an over-the-counter iron supplement.

Start with one action

If you’ve just been told your iron is low, start by asking:

Do we know why?

Replacing iron is important.

But if you continue losing blood, cannot absorb iron properly or have another underlying condition, the deficiency may simply return.

Your next step may be as simple as improving diet and taking oral iron.

Or it may involve investigating heavy periods, gastrointestinal blood loss, coeliac disease, pregnancy, blood donation or another cause.

The right plan depends on the person.

The Bottom Line

Iron deficiency means your body’s iron stores are running low.

You can have iron deficiency before you develop anaemia, and ferritin is usually the most useful blood test for assessing stored iron.

Treatment often involves oral iron. Intravenous iron is useful in selected circumstances.

But replacing iron is only half the job.

If your ferritin is low, the most important question is often not “Which iron supplement should I take?” but “Why am I iron deficient in the first place?”

Finding and treating that cause helps prevent the problem from simply coming back.

Related Q Guides

• Healthy Diet
• Cancer Screening: What You Need and When
• Menopause and Midlife Health
• Heavy Periods: When to Get Them Checked
• Welcome to My Practice