What Is Iron Deficiency?

What Does Iron Do in Your Body?

Iron is a mineral your body cannot function without. Its most familiar role is making haemoglobin — the protein inside red blood cells that carries oxygen from your lungs to every part of your body. But iron does much more than that:

• Muscle function — iron is part of myoglobin, the protein that stores oxygen inside muscle cells. When iron is very low, muscles cannot work properly — which is why iron deficiency causes weakness and fatigue beyond just breathlessness.
• Energy production — iron plays a central role in the system inside cells that generates energy. This is why you can feel exhausted with low iron even before your haemoglobin has dropped to the anaemia threshold.
• Thyroid function — iron is needed to make thyroid hormones, so low iron can affect how your thyroid works.
• Immune system — iron is required for your immune cells to multiply and fight infection effectively.
• Brain chemistry — iron is involved in making several important brain chemicals, including dopamine and serotonin. This is likely why iron deficiency can affect your mood and ability to concentrate.

About 70% of your body's iron is locked up in haemoglobin and myoglobin. The rest is stored mainly in the liver, spleen, and bone marrow as a protein called ferritin. A small amount travels through the blood attached to a carrier protein called transferrin. Your body regulates iron levels by controlling how much it absorbs from food — normally just 1–2 mg per day — because it has no good way of getting rid of excess iron.

How Iron Deficiency Develops — Three Stages

Iron deficiency does not happen overnight. It builds gradually through three stages as your iron stores are progressively depleted:

Stage 1: Iron stores start to fall

Your ferritin (stored iron) drops below normal, but your blood iron levels and haemoglobin are still fine. You may have no symptoms at this stage — or just mild fatigue. Even without anaemia, a low ferritin alone is worth treating, and the underlying cause still needs to be found.

Stage 2: Your body struggles to make healthy red cells

Your iron stores are essentially empty. Your blood iron drops, your body starts producing more carrier protein (transferrin) to try to grab iron from wherever it can, and your bone marrow begins making smaller, paler red cells. Your haemoglobin has not yet fallen enough to qualify as anaemia, but things are heading that way.

Stage 3: Iron deficiency anaemia

Your haemoglobin drops below normal — below 130 g/L in men and 120 g/L in women (World Health Organization criteria). Symptoms become clear or worsen: tiredness, breathlessness on exertion, pallor, poor concentration, and palpitations. A blood test at this stage will show small, pale red blood cells — what doctors call a microcytic, hypochromic picture.

Understanding Your Blood Test Results

If your doctor has checked your iron levels, these are the key results and what they mean:

• Ferritin — your iron store level. Normal is roughly 30–300 µg/L in men and 15–200 µg/L in women. Below 30 µg/L strongly suggests iron deficiency; below 15 µg/L is diagnostic. One important caveat: ferritin rises when your body is fighting inflammation or infection, so it can appear normal even when your iron stores are actually low.
• Serum iron — the iron currently circulating in your blood. Normal is about 10–30 µmol/L. Low in iron deficiency.
• Total iron-binding capacity (TIBC) — how much iron your blood proteins could carry if fully loaded. Normal is about 45–72 µmol/L. In iron deficiency, this rises as your body produces more carrier protein to try to scavenge iron.
• Transferrin saturation — how full those carriers actually are, expressed as a percentage. Normal is 20–45%. Below 16% suggests your bone marrow is not getting enough iron to make healthy red cells.
• Haemoglobin (Hb) — normal is 130–175 g/L in men and 120–155 g/L in women.

What Causes Iron Deficiency?

Iron deficiency comes from one or more of four things: not getting enough iron in your diet, your gut not absorbing it properly, your body needing more iron than usual, or losing blood.

Not enough iron in the diet

A diet low in iron-rich foods — particularly red meat, poultry, and fish — can cause iron deficiency over time. Groups most at risk include strict vegetarians and vegans (plant-based iron is harder for the body to absorb), babies who are still exclusively breastfed past six months without iron-containing foods, and elderly people with a poor appetite or very restricted diet.

Your gut not absorbing iron properly

Even if you are eating enough iron, certain conditions can stop it from being absorbed properly:

• Coeliac disease — this damages the lining of the small bowel, dramatically reducing iron absorption. Iron deficiency anaemia is sometimes the only sign of undiagnosed coeliac disease.
• Gastric bypass surgery — bypasses the part of the small bowel (the duodenum) where most iron absorption happens.
• Helicobacter pylori infection — a common stomach bug that reduces stomach acid and impairs iron absorption; treating the infection can help iron levels recover.
• Heartburn medications (PPIs) — proton pump inhibitors reduce stomach acid, which your body needs to convert dietary iron into the form it can absorb.
• Atrophic gastritis — a condition where the stomach lining is thinned and produces little acid, reducing iron absorption.

Your body needing more iron than usual

During pregnancy, your iron needs increase substantially — your blood volume expands and the baby and placenta both need iron. Rapidly growing adolescents and endurance athletes can also run into iron deficiency for this reason. Women of reproductive age need more iron than men because of monthly blood loss with periods.

Blood loss — the most important cause in adults

In adults, losing blood is by far the most significant cause of iron deficiency — and often the most important one to find. Every millilitre of blood contains about 0.5 mg of iron. Losing as little as 2–4 ml of blood per day is enough to slowly deplete your iron stores over time.

Sources of blood loss include:

• The gut — stomach ulcers, oesophagitis (inflammation in the food pipe), stomach cancer, bowel cancer, bowel polyps, haemorrhoids, diverticular disease, and inflammatory bowel disease. Crucially, gut bleeding is often occult — invisible to the naked eye — meaning you may not notice it at all.
• Periods — the most common cause of iron deficiency in premenopausal women.
• The urinary system — blood in the urine from bladder or kidney conditions.
• Repeated blood donation or therapeutic venesection (having blood removed as a treatment)
• Lung conditions causing bleeding into the lung tissue — rare.

The Most Important Point: Men and Postmenopausal Women

This point matters enormously. Bowel cancer often bleeds slowly and silently into the gut — the blood mixes with waste and you cannot see it. You can feel completely well and have no bowel symptoms at all. For some people, iron deficiency picked up on a blood test is the only early clue that cancer is present. Finding it at that stage, before symptoms develop, dramatically improves the chances of a cure.

How Iron Deficiency Is Investigated

Once iron deficiency is identified on your blood test, the following investigations are typically arranged depending on your situation:

• Full blood examination — confirms whether anaemia is also present and shows what your red blood cells look like.
• Iron studies — ferritin, serum iron, TIBC, and transferrin saturation — to fully characterise the deficiency.
• Coeliac blood test — checks for coeliac disease (an immune condition causing damage to the small bowel lining), which is a common and often silent cause of poor iron absorption.
• Faecal occult blood test (FOBT) — a stool test that detects microscopic blood that is invisible to the eye.
• Colonoscopy — a camera examination of the lower bowel, looking for bowel cancer, bowel polyps, diverticular disease, or abnormal blood vessels. Recommended for all men and postmenopausal women with unexplained low iron.
• Gastroscopy — a camera examination of the oesophagus, stomach, and the first part of the small bowel, looking for ulcers, stomach cancer, inflammation, or coeliac disease (biopsies are taken from the duodenum).
• H. pylori testing — a breath test or stool test for the stomach bug Helicobacter pylori, which can impair iron absorption.
• Capsule endoscopy — a swallowed camera that photographs the entire small bowel, used when upper and lower camera tests have not found the cause.

Can Medications Cause Iron Deficiency?

Yes. Some common medications can contribute. Your full medication list is always reviewed as part of the investigation:

• Anti-inflammatory painkillers (NSAIDs) — such as aspirin and ibuprofen, can cause irritation, ulcers, or inflammation in the stomach lining, leading to slow ongoing blood loss that quietly depletes iron over time.
• Blood thinners — warfarin, rivaroxaban, and apixaban do not directly cause iron deficiency, but they increase bleeding risk, which can turn a small pre-existing bowel lesion into a more significant source of blood loss.
• Heartburn medications (PPIs) — reduce stomach acid, which is needed to help your gut absorb iron from food.

Mentioning all the medications and supplements you take — including over-the-counter painkillers — helps your doctor build the full picture.

Treating Iron Deficiency

Treatment has two equally important parts: replacing the iron and finding and treating the cause. Restoring your iron without finding the source of the problem is not enough — your stores will simply fall again if blood loss continues.

Your iron can be replaced in three ways:

• Iron tablets — ferrous sulphate or ferrous gluconate are inexpensive and effective when your gut can absorb iron well. Common side effects include constipation, nausea, and dark stools. Taking iron with vitamin C helps absorption.
• Intravenous iron infusion — used when tablets cause intolerable side effects, your gut cannot absorb iron properly, or your iron needs to be restored quickly (for example, before surgery). See our separate article on iron infusions for more.
• Blood transfusion — reserved for severe symptomatic anaemia (haemoglobin below about 70–80 g/L) or when the situation is urgent.

Frequently Asked Questions