Iron Deficiency Anaemia: Finding the Cause, Not Just Replacing the Iron

Iron deficiency is the commonest cause of anaemia worldwide and one of the most frequently missed: it is easy to treat, but treating it without finding the cause can miss a bleeding ulcer or a colorectal cancer. The 2021 British Society of Gastroenterology guideline on the management of adult iron deficiency anaemia is explicit on this point — replacement and investigation go together.

Confirming iron deficiency

Iron deficiency is diagnosed on blood tests: a low haemoglobin with microcytic, hypochromic red cells, and a low ferritin, which is the most useful single measure of iron stores. Inflammation raises ferritin, so a normal or high ferritin does not exclude deficiency in someone with infection, inflammatory disease or cancer — in those situations transferrin receptor or the reticulocyte haemoglobin content helps. A therapeutic trial of iron with a rise in haemoglobin after a few weeks is a practical confirmation in ambiguous cases. The commonest confusion is with anaemia of chronic disease and with thalassaemia trait, both of which can look microcytic.

Replacing iron

Oral iron is first-line, inexpensive and effective: a single daily dose of ferrous sulfate or an equivalent — and, increasingly, alternate-day dosing, which improves absorption and reduces side effects by allowing hepcidin levels to fall between doses. Taking it with vitamin C and on an empty stomach improves absorption, but tolerability matters more than perfection, so a lower dose or a different formulation is often the practical answer. Side effects — nausea, constipation, dark stools — are common and cause many patients to stop. Intravenous iron is used when oral iron is not tolerated or not absorbed — coeliac disease, inflammatory bowel disease, after gastric bypass — when anaemia is severe or symptomatic, when there is ongoing blood loss that outpaces oral replacement, or when time matters, such as before surgery. Modern IV formulations can replace the full deficit in one or two infusions with a low risk of serious reaction.

Finding the cause

In men and in postmenopausal women, iron deficiency is assumed to be gastrointestinal until proved otherwise, and upper and lower gastrointestinal endoscopy is recommended — coeliac serology and, where appropriate, capsule endoscopy or enteroscopy for cases where both scopes are negative. In premenopausal women, heavy menstrual loss is the commonest cause, and a trial of iron with gynaecological assessment is reasonable, but gastrointestinal investigation is still indicated if symptoms, age, severity or failure to respond suggest it. Other causes to consider include dietary deficiency, blood donation, chronic kidney disease, Helicobacter pylori infection, and medications such as long-term NSAIDs or aspirin.

After treatment

Haemoglobin should rise by roughly 10–20 g/L within a few weeks and normalise within about two months; a failure to respond prompts a review of adherence, absorption, ongoing loss and the diagnosis itself. Iron stores should be repleted, not just the haemoglobin corrected — continuing oral iron for about three months after normalisation is standard. After successful treatment, monitoring is periodic rather than indefinite, and any recurrence of anaemia warrants reinvestigation rather than simply another prescription.

Practical advice

Do not start self-prescribed iron before investigation if you are in a group where endoscopy is indicated — it can mask the finding without treating the cause. Separate iron from calcium, tea, coffee and antacids. Expect black stools and constipation; mention them rather than stopping silently. And if you are offered iron without any discussion of why you are deficient, ask what investigation is planned to find the source.