What Causes Iron Deficiency? A Doctor Explains

Heavy periods are the leading cause, and the most overlooked

Iron deficiency occurs when your body loses, uses, or fails to absorb more iron than it takes in. The most common causes in adults are heavy menstrual bleeding, pregnancy, poor gut absorption, and chronic blood loss from the gastrointestinal tract. Diet plays a smaller role than most people assume.

Understanding the cause matters because the right treatment depends on it. Replacing iron through an infusion corrects the deficiency quickly, but if the underlying reason is not addressed, levels will fall again.

Heavy periods are the leading cause in women of reproductive age

Menorrhagia (heavy menstrual bleeding) is the single most common reason women develop iron deficiency in the UK, according to NICE CKS on iron deficiency anaemia. Each cycle can remove significantly more iron than diet replaces, and the deficit compounds month after month.

Many women are told their periods are "normal" when they are actually unusually heavy. If you are changing protection every one to two hours, passing clots, or bleeding for more than seven days, that is worth investigating. Our page on iron infusion for heavy periods explains how we approach this at IronOx.

Pregnancy and the postpartum period create a sharp increase in demand

During pregnancy your blood volume expands significantly to support the placenta, the growing baby, and your own increased red cell mass. Many women enter pregnancy with borderline stores and become frankly deficient by the third trimester.

Blood loss during delivery then depletes stores further. Postnatal fatigue is often dismissed as a normal part of new parenthood, but low iron is a treatable and very common cause. Our blog post on low iron after having a baby covers this in more detail, and our iron infusion in pregnancy page explains the treatment options.

Poor gut absorption blocks iron even when intake is adequate

You can eat a diet full of red meat, lentils, and leafy greens and still become iron deficient if your gut cannot absorb it properly. Conditions that damage the small intestine or alter stomach acid reduce iron uptake significantly:

• Coeliac disease (often undiagnosed)
• Crohn's disease or ulcerative colitis
• Gastric bypass or other bariatric surgery
• Long-term use of proton pump inhibitors (PPIs)
• Helicobacter pylori infection
• Atrophic gastritis

This is also why oral iron tablets fail for a significant proportion of patients. The NHS page on iron deficiency anaemia acknowledges that tablets are not always effective, and the British Society of Gastroenterology guidelines recommend IV iron when oral treatment has failed or is not tolerated. Our post on why iron tablets stop working goes into the absorption problem in more detail.

Gastrointestinal blood loss is a cause that must not be missed

Slow, ongoing blood loss from the gut is a common cause of iron deficiency in men and in postmenopausal women. It can come from peptic ulcers, oesophagitis, bowel polyps, haemorrhoids, or, in rarer cases, colorectal cancer. The bleeding is often invisible to the patient.

This is why the British Society of Gastroenterology guidelines recommend that iron deficiency in men of any age and in postmenopausal women should prompt investigation of the gastrointestinal tract before treatment is started. If you have not had a cause identified, your GP or a gastroenterologist should be involved.

Vegetarian and vegan diets increase the risk, but are rarely the whole story

Plant-based iron (non-haem iron) is absorbed far less efficiently than haem iron from meat. Phytates in wholegrains and legumes, and polyphenols in tea and coffee, reduce absorption further. Vegetarians and vegans do have higher rates of iron deficiency, but in most cases a poor diet is a contributing factor rather than a standalone cause.

If you follow a plant-based diet and your ferritin is low, it is still worth checking for the other causes above rather than assuming diet explains everything.

Intense exercise raises iron requirements

Endurance athletes, particularly runners, lose iron through sweat, through foot-strike haemolysis (red cell destruction from repeated impact), and through increased gut motility. Female athletes are especially vulnerable because they combine high demand with menstrual losses. Fatigue and poor recovery in athletes are sometimes labelled as overtraining when low ferritin is the actual problem.

What to do if you have identified a likely cause

Knowing the cause helps you choose the right next step. If absorption is the problem, oral tablets will not solve it. If blood loss is ongoing, iron replacement alone is not enough. If you are postpartum, pregnant, or heading into surgery, timing matters.

At IronOx Clinic in Cardiff, a free free telephone consultation includes a review of your blood results and a conversation about the most likely cause. We use ferric carboxymaltose, the NICE-recommended IV iron preparation, and most patients receive either the IronOx 500 (500mg, £699) or IronOx 1000 (1000mg, £899), each including a complimentary hyperbaric oxygen therapy session. Prices are correct as of 2026 and are subject to change; please check with the clinic for the latest figures.

For context on how low your ferritin needs to be before treatment is warranted, our pillar post on what is dangerously low ferritin levels covers the thresholds in clinical detail.

NHS Wales haematology waiting times vary and can be lengthy. A private consultation at IronOx carries no deposit and no obligation, and we can usually see you within days.

The causes at a glance

Sources

• NICE, Heavy menstrual bleeding: assessment and management (NG88): a full blood count for all women with heavy menstrual bleeding; routine serum ferritin is not part of that pathway.
• British Society of Gastroenterology, Guidelines for the management of iron deficiency anaemia in adults (2021): the UK specialist guideline: why the cause of iron deficiency must be established rather than the deficiency simply replaced.
• NHS, Iron deficiency anaemia: the NHS overview of symptoms, the full blood count used to diagnose it, and the pathway of establishing the cause before treating with iron tablets.
• NICE CKS, Anaemia: iron deficiency: the primary care assessment and management pathway UK GPs work to, including investigation of the underlying cause and when specialist referral is indicated.
• NHS, Iron (vitamins and minerals): the UK reference intakes quoted above: 8.7 mg a day for men aged 19 and over, 14.8 mg a day for women aged 19 to 49, and 8.7 mg a day for women aged 50 and over.

Reviewed by Dr Ros Jabar, GMC 7265326.