Current guidelines on iron deficiency anemia follow three steps: confirm the anemia with a blood count, prove iron deficiency with ferritin and iron studies, then find and treat the cause while replacing iron. Oral iron is first-line, intravenous iron is used when tablets fail or are unsuitable, and follow-up checks that hemoglobin rises and stores refill. Skipping the search for a cause is the most common mistake.
This guide sets out the diagnostic cut-offs, the work-up for common causes, treatment choices, and how to monitor response. For related conditions, see our main anemia guide.
What Iron Deficiency Anemia Is
Iron deficiency anemia is the most common hematological condition worldwide. Iron is the core of hemoglobin, the protein that lets red blood cells carry oxygen. Without enough iron, the bone marrow produces red cells that are small (microcytic) and pale (hypochromic).
Deficiency develops in stages. First, iron stores in the liver, spleen, and marrow run down. Next, the supply to the marrow falls. Only at the end does hemoglobin drop, which is why ferritin can be low for months before a blood count looks abnormal.
Causes and Risk Factors
Iron deficiency happens when losses or needs exceed what the body absorbs. The main causes are:
- Blood loss: heavy menstrual bleeding and gastrointestinal bleeding, from ulcers, inflammation, polyps, or bowel cancer.
- Increased demand: pregnancy, infancy, and adolescent growth spurts.
- Poor absorption: celiac disease, Helicobacter pylori infection, gastric surgery, and long-term acid-suppressing medicines.
- Low intake: restrictive or poorly planned plant-based diets.
- Other losses: frequent blood donation and, in some athletes, losses from intense training.
Diagnosis: Tests and Cut-Offs
Typical symptoms are fatigue, pallor, breathlessness on exertion, headaches, restless legs, and pica, a craving for ice or non-food items. Severe anemia can cause a fast heart rate. Symptoms alone cannot confirm the diagnosis, so testing is essential.
| Test | Usual finding in iron deficiency | Notes |
|---|---|---|
| Hemoglobin | Below 13 g/dL (men), 12 g/dL (women), 11 g/dL (pregnancy) | Standard WHO thresholds for anemia |
| MCV (red cell size) | Below 80 fL | May be normal early or with a mixed deficiency |
| Ferritin | Below 15 ng/mL is diagnostic | Many clinicians use below 30 ng/mL; higher cut-offs apply with inflammation |
| Transferrin saturation | Below 20% | Useful when ferritin is borderline |
| Blood film | Small, pale cells, pencil cells | Helps exclude other causes |
Ferritin rises during inflammation, infection, liver disease, and cancer, so a “normal” ferritin does not always rule out deficiency in those settings. Here, transferrin saturation helps.
Low iron can occur alongside other cytopenias; our article on low hemoglobin and platelets covers when both counts fall together.
Finding the cause
Every diagnosis of iron deficiency should prompt the question: why? Guidelines generally recommend:
- Celiac disease screening for most adults with iron deficiency.
- Upper and lower gastrointestinal investigation (gastroscopy and colonoscopy) for men and postmenopausal women, since bowel cancer and other bleeding sources must be excluded.
- Menstrual history for premenopausal women, with gut investigation if there are bowel symptoms, a family history of bowel cancer, or poor response to iron.
- Urine testing for blood, as urinary tract bleeding is an occasional cause.
Management Guidelines
Treatment aims to correct hemoglobin, refill iron stores, and deal with the underlying cause.
Oral iron: first-line
Ferrous sulfate, ferrous fumarate, and ferrous gluconate are all effective. One tablet daily, or on alternate days, is usually as effective as multiple daily doses and causes fewer side effects. Take it on an empty stomach if tolerated, with water or vitamin C, and away from tea, coffee, calcium, and antacids.
Constipation, nausea, and dark stools are common. Switching preparation or dosing on alternate days often helps before giving up on tablets.
Intravenous iron
IV iron is recommended when oral iron is not tolerated or not absorbed, when losses continue faster than tablets can replace, in inflammatory bowel disease or chronic kidney disease, and late in pregnancy when rapid correction is needed. Modern preparations can replace a large dose in one or two short visits. Severe allergic reactions are rare, and infusions are given where staff can manage them.
Transfusion
Blood transfusion is reserved for severe symptomatic anemia or active heavy bleeding. It is not a substitute for iron replacement.
Diet
Diet supports but rarely corrects established deficiency. Heme iron from red meat, poultry, and fish is absorbed best; plant sources such as beans, lentils, tofu, and fortified cereals are absorbed better with vitamin C.
Monitoring and Follow-Up
| Timing | What to check | What it tells you |
|---|---|---|
| 2 to 4 weeks | Hemoglobin | A rise of around 2 g/dL within about four weeks shows a good response |
| End of treatment | Hemoglobin and ferritin | Continue iron for about 3 months after hemoglobin normalizes |
| Every few months, then yearly | Blood count, ferritin | Detects recurrence, especially with ongoing losses |
A poor response usually means ongoing bleeding, non-adherence, malabsorption, or a second diagnosis. It should trigger review rather than simply more tablets. Our article on who treats iron deficiency anemia explains when care moves from the family doctor to a specialist.
Special Situations
In pregnancy, demand rises sharply, and screening is routine; see our guide to anemia in pregnancy. In chronic kidney disease, heart failure, and inflammatory bowel disease, IV iron is used more often. For how these recommendations fit into broader practice, see our overview of anemia guidelines.
Key Takeaways
- Confirm anemia with a blood count, then prove iron deficiency with ferritin and, where needed, transferrin saturation.
- Always look for the cause: heavy periods, gut bleeding, celiac disease, and pregnancy top the list.
- Men and postmenopausal women with iron deficiency need gastrointestinal investigation.
- Oral iron, once daily or on alternate days, is first-line; IV iron is for intolerance, malabsorption, ongoing losses, or urgency.
- Recheck hemoglobin within a few weeks, and continue iron for about three months after it normalizes.
In my practice, the patients who do best are those whose cause is found and fixed early. Replacing iron without asking why it ran low only postpones the problem.
Frequently Asked Questions
What ferritin level means iron deficiency?
A ferritin below 15 ng/mL confirms iron deficiency, and many clinicians treat below 30 ng/mL when the picture fits. With active inflammation, deficiency can exist at higher levels, so doctors interpret it alongside transferrin saturation.
How long should I take iron tablets?
Usually until hemoglobin is normal and then for about three more months to refill stores. Your doctor will check levels before stopping.
Why would I need a colonoscopy for low iron?
In men and women after menopause, slow bleeding from the gut is a common hidden cause, including bowel cancer. Investigation finds problems early, when they are most treatable.
Is IV iron better than tablets?
It works faster and bypasses the gut, but tablets work well for most people and are simpler. IV iron is chosen for specific reasons, such as intolerance, malabsorption, or the need for rapid correction.
