Iron Deficiency: Symptoms, Causes and How It's Treated

Information for patients who have been told their iron is low, or who suspect it might be. Bring this along to talk through with your GP.

Iron deficiency is the most common nutrient deficiency we see in general practice — and one of the most commonly missed, because its symptoms are easy to put down to a busy life. It is estimated that around 8% of premenopausal women in Australia have biochemical iron deficiency, and many have no idea (1). This guide explains what iron does, the symptoms low iron causes (even before anaemia develops), why finding the cause matters as much as fixing the level, and how it is treated — from food to tablets to an iron infusion.

Numbers in brackets link to the peer-reviewed source for that statement, listed in full under References.

What iron does

Iron is the working heart of haemoglobin, the protein in red blood cells that carries oxygen around your body. It is also needed by muscles (in myoglobin), by the enzymes your cells use to make energy, and by the brain. That is why the symptoms of low iron reach well beyond “low blood count” (2)(3).

Your body stores iron — mostly measured by a blood protein called ferritin — and draws those stores down before anything shows on a standard blood count. That leads to a key distinction:

• Iron deficiency — your stores are depleted (low ferritin), but haemoglobin may still be normal. Symptoms can already be present.
• Iron-deficiency anaemia — the deficiency has progressed far enough that haemoglobin has fallen. Symptoms are usually more pronounced.

You do not need to be anaemic for low iron to be worth treating: clinical and functional impairments can occur in the absence of anaemia (2).

Symptoms of low iron

Symptoms build gradually and are easy to normalise (2)(3). Common ones include:

• Fatigue and low energy — the classic symptom, often with reduced exercise tolerance.
• Brain fog — difficulty concentrating, feeling flat or irritable.
• Hair shedding and brittle nails.
• Restless legs, particularly at night.
• Cravings for ice or other non-food substances (called pica) — a curious but well-recognised sign.
• With anaemia: paleness, breathlessness on exertion, dizziness and a racing heart.

What causes iron deficiency?

There are four broad mechanisms, and working out which applies to you is a core part of the consultation:

1. Blood loss

Every millilitre of blood takes iron with it. Heavy menstrual periods are far and away the most common cause in premenopausal women. The other major source is the gastrointestinal tract — ulcers, inflammation, regular anti-inflammatory or aspirin use, polyps, and occasionally bowel cancer (4). Blood loss from the gut is often invisible, which is exactly why unexplained iron deficiency should never simply be “topped up” without asking where the iron went.

2. Increased need

Pregnancy and breastfeeding, childhood and adolescent growth spurts, and heavy endurance training all raise iron requirements — sometimes beyond what diet can supply (1).

3. Not enough coming in

Iron from meat (haem iron) is absorbed much more readily than iron from plants (non-haem iron). People who eat little or no red meat — including vegetarians and vegans — can meet their needs with planning, but have less buffer, especially when needs rise or losses increase (1).

4. Poor absorption

Iron is absorbed in the upper small bowel, and anything that disrupts this can cause deficiency: coeliac disease — which should be screened for routinely in iron deficiency (2) — stomach or weight-loss surgery, Helicobacter pylori infection, and long-term acid-suppressing medication (3). Tea, coffee and calcium also blunt absorption of iron eaten at the same time — a practical point that matters for treatment too.

Finding the cause matters

Iron deficiency is a finding, not a final diagnosis. For premenopausal women with heavy periods and a diet light on red meat, the explanation is often clear — and managing the periods can be part of the treatment. But in men and postmenopausal women, iron deficiency is unusual enough that guidelines recommend evaluation of the bowel to look for a source of blood loss, even when there are no gut symptoms (4). Occasionally, low iron is the first clue to a bowel cancer at its most treatable stage. Your GP will judge what investigation, if any, your situation calls for.

How iron deficiency is diagnosed

A simple blood test. Ferritin is the key measure of your iron stores, usually checked with a full blood count and sometimes full iron studies (including transferrin saturation) (5).

What ferritin actually tells us — the bank vault

The only way to know your true iron store level is to perform a bone marrow biopsy (6). For some surprising reason, that has never been a popular request! So instead we use ferritin as a loose guide to the stores.

Imagine you want to know how much money a bank has in its vault — but you cannot see inside, because the bank will not let you. So you stand outside and count the armoured vans coming and going. Plenty of armoured vans, and you assume the bank is rich and successful. Hardly any vans, and you conclude there probably is not much money in the vault.

This is exactly how we gauge iron stores. Ferritin is essentially a transport and storage molecule for iron in the blood — and just as in the vault scenario, the body does not usually bother making many transport molecules for iron if there is not much iron around. It is a reasonable inference. It is not a direct measurement.

And, like any inference, it can mislead. Many things make ferritin “look good” in the presence of true iron deficiency — infection, inflammation, liver disease, recent illness — because ferritin also rises as part of the body’s stress response (6). This is the single most important catch in reading an iron study: a normal-looking ferritin in someone who is unwell does not rule out iron deficiency. Reading ferritin without a bone marrow biopsy is, frankly, an inexact science.

So what number is too low?

When ferritin was tested directly against bone marrow examination — the gold standard — in a large overview of the published studies, it outperformed every other blood test available (area under the ROC curve 0.95). Crucially, that analysis treated ferritin as a continuum rather than a single cut-off: the lower the ferritin, the more strongly iron deficiency is indicated, and the interpretation shifts again when inflammatory, liver or malignant disease is present (6).

That is why doctors read ferritin as a graded scale rather than a pass/fail line — and why the threshold that matters depends on who you are and what symptoms you have:

• Around 15 or below — iron stores are essentially empty (7).
• Below about 30 — iron deficiency in an otherwise well adult (4)(5).
• 30–50 — deficiency is still quite likely, especially if you have symptoms. This band is where much of the fatigue research sits: randomised trials of iron in non-anaemic women with a ferritin under 50 have shown real improvements in fatigue (8)(9).
• Up to about 75 — for restless legs syndrome specifically, international guidelines say oral iron is possibly effective in people with a ferritin of 75 or below, and an infusion can help at levels considerably higher again (10).
• Under 100 — where inflammation or chronic disease is in play, the bar moves up substantially, precisely because the illness itself inflates the number. The landmark heart-failure trials FAIR-HF and CONFIRM-HF defined iron deficiency as a ferritin under 100, or 100–300 when transferrin saturation was under 20% (11)(12).

So a ferritin of 45 is not “normal” simply because it clears the bottom of the reference range. If you are tired, or your legs will not settle at night, that number is worth acting on — and it is why we treat the patient in front of us, not the lab report. If the picture is murky, your doctor may repeat the test when you are well, or use additional markers.

Does replacing iron actually improve energy?

Yes. Importantly, this has been shown in people who are iron deficient but not anaemic — that is, in exactly the situation where a blood count looks fine and only the ferritin is low. The benefit is consistently found for how people feel, and the effect is largest in those who start lowest:

• Oral iron, ferritin under 50. In a randomised controlled trial of non-anaemic menstruating women with a ferritin under 50, fatigue scores fell by 47.7% on iron versus 28.8% on placebo (a between-group difference of 18.9 percentage points, p = 0.02) (8).
• Oral iron, ferritin under 50. In an earlier double-blind trial of women with unexplained fatigue, fatigue fell by 29% on iron versus 13% on placebo after one month — and it was specifically the women with a ferritin of 50 or under who improved (9).
• Pooled evidence. A systematic review of randomised trials in non-anaemic iron-deficient adults found iron supplementation significantly reduced self-reported fatigue (standardised mean difference −0.38, 95% CI −0.52 to −0.23, across 4 trials and 714 participants) — though it did not improve objective measures of physical capacity such as maximal oxygen consumption (13). The gain is real, and it is a subjective one: people feel better.
• Iron infusion, lowest stores. In a placebo-controlled trial of intravenous iron in non-anaemic women with a ferritin of 50 or under, the overall six-week benefit fell just short of significance — but in the subgroup with genuinely depleted stores (ferritin 15 or under), 82% of iron-treated women reported improved fatigue, versus 47% on placebo (7).
• Iron infusion (PREFER). In fatigued, iron-deficient women, a single infusion improved fatigue in 65.3% versus 52.7% on placebo, and 33.3% halved their fatigue score versus 16.4% on placebo (14).
• Higher ferritins, with chronic illness. In heart-failure patients defined as iron deficient at a ferritin under 100 (or up to 300 with a low transferrin saturation), intravenous iron significantly improved fatigue score, symptom class, patient global assessment and quality of life (12).

Taken together: the fatigue benefit of correcting iron is real, is well demonstrated at ferritin levels well above the “empty stores” mark of 15, and grows as the starting ferritin falls. This is why a tired patient with a ferritin in the 30s or 40s — or a ferritin under 75 with restless legs, or under 100 alongside a chronic illness — is worth treating rather than reassuring.

Treatment: the ladder

The right treatment depends on the cause, how low you are, how symptomatic you are, and how quickly iron needs to be restored. Alongside every option below, the cause itself is addressed — otherwise the deficiency simply returns.

1. Food first

For mild deficiency with a fixable dietary gap: red meat is the richest source of well-absorbed haem iron; chicken and fish contribute too. Plant sources — legumes, tofu, wholegrains, green leafy vegetables, nuts and fortified cereals — are worthwhile but less well absorbed. Two practical boosters: pair plant iron with vitamin C (capsicum, citrus, tomato), and keep tea and coffee away from iron-rich meals (1). Diet alone rebuilds stores slowly, so it usually supports rather than replaces the options below.

2. Oral iron — with modern dosing

Iron tablets are effective, inexpensive and first-line for most people (2). Two evidence-based points make them work better:

• Every second day, not every day. After a dose of iron, your body raises hepcidin — a hormone that blocks iron absorption — for about a day. Trials in iron-depleted women showed alternate-day dosing absorbs iron more efficiently than daily dosing (15). Fewer tablets, better absorbed, usually gentler on the stomach.
• Expect the gut to have opinions. Traditional ferrous sulfate causes gastrointestinal side effects — constipation, nausea, dark stools — in a substantial proportion of people (16). If tablets upset you, don’t give up on treatment: switching preparation, taking iron with a small amount of food, or moving to alternate-day dosing all help, and your GP can adjust the plan.

Treatment continues beyond the point of “feeling better”: guidelines advise continuing iron for around three months after haemoglobin normalises to refill your stores, with follow-up blood tests to confirm (4)(5).

3. Intravenous iron — the infusion

When tablets are not the answer, an iron infusion delivers a full course of iron directly into a vein in a single visit. It is considered when:

• oral iron isn’t tolerated despite adjusting dose and preparation;
• iron isn’t being absorbed — for example coeliac disease, or after stomach/weight-loss surgery;
• ongoing losses (such as heavy periods) outpace what tablets can replace;
• iron needs to be restored quickly — before surgery, in later pregnancy, or when anaemia is significant and symptomatic.

At The Local Doctor, an iron infusion is a single one-hour, nurse-led visit with your doctor present, at both our Ivanhoe and Diamond Creek clinics. Your doctor provides a script for the iron, which you purchase from the pharmacy beforehand. Full details — including what to expect on the day and the current cost — are on our iron infusion page, and all consultation and procedure fees are listed on our fees pages.

A word of caution about self-supplementing

Please don’t take iron “just in case”. Excess iron causes gut side effects, can mask a cause that needed finding, and is genuinely harmful for people with iron-overload conditions such as hereditary haemochromatosis. And a household safety note worth repeating: keep iron tablets well out of reach of children — they are a common cause of poisoning in young children, and small bodies handle iron overdose badly (1)(5). Confirm deficiency with a blood test, treat it properly, and recheck.

When to see your GP

If you are persistently tired, notice the symptoms above, are a blood donor, have heavy periods, follow a vegetarian or vegan diet, are pregnant or planning pregnancy, or have a family history of bowel problems — a simple blood test will give a clear answer. Book a consultation at our Ivanhoe or Diamond Creek clinics and we will take it from there.

References

Numbered in the order they appear above. Links open the PubMed record, the article’s permanent (DOI) page, or the guideline.

1. Iron deficiency — adults — Better Health Channel, Victorian Government.
2. Iron deficiency — Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. The Lancet, 2021;397(10270):233–248.
3. Iron-deficiency anemia — Camaschella C. New England Journal of Medicine, 2015;372(19):1832–1843.
4. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults — Snook J, Bhala N, Beales ILP, et al. Gut, 2021;70(11):2030–2051.
5. Correcting iron deficiency — Baird-Gunning J, Bromley J. Australian Prescriber, 2016;39(6):193–199.
6. Laboratory diagnosis of iron-deficiency anemia: an overview — Guyatt GH, Oxman AD, Ali M, Willan A, McIlroy W, Patterson C. Journal of General Internal Medicine, 1992;7(2):145–153.
7. Intravenous iron for the treatment of fatigue in nonanemic, premenopausal women with low serum ferritin concentration — Krayenbuehl PA, Battegay E, Breymann C, Furrer J, Schulthess G. Blood, 2011;118(12):3222–3227.
8. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial — Vaucher P, Druais PL, Waldvogel S, Favrat B. CMAJ, 2012;184(11):1247–1254.
9. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial — Verdon F, Burnand B, Fallab Stubi CL, et al. BMJ, 2003;326(7399):1124.
10. Evidence-based and consensus clinical practice guidelines for the iron treatment of restless legs syndrome/Willis-Ekbom disease in adults and children: an IRLSSG task force report — Allen RP, Picchietti DL, Auerbach M, et al. Sleep Medicine, 2018;41:27–44.
11. Ferric carboxymaltose in patients with heart failure and iron deficiency (FAIR-HF) — Anker SD, Comin Colet J, Filippatos G, et al. New England Journal of Medicine, 2009;361(25):2436–2448.
12. Beneficial effects of long-term intravenous iron therapy with ferric carboxymaltose in patients with symptomatic heart failure and iron deficiency (CONFIRM-HF) — Ponikowski P, van Veldhuisen DJ, Comin-Colet J, et al. European Heart Journal, 2015;36(11):657–668.
13. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials — Houston BL, Hurrie D, Graham J, et al. BMJ Open, 2018;8(4):e019240.
14. Evaluation of a single dose of ferric carboxymaltose in fatigued, iron-deficient women — PREFER, a randomized, placebo-controlled study — Favrat B, Balck K, Breymann C, et al. PLoS One, 2014;9(4):e94217.
15. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials — Stoffel NU, Cercamondi CI, Brittenham G, et al. The Lancet Haematology, 2017;4(11):e524–e533.
16. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis — Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JJ. PLoS One, 2015;10(2):e0117383.