Iron Deficiency and Ferritin: Why a "Normal" Result Can Still Be Too Low
A ferritin result inside the lab’s “normal” range can still be too low for you. UK labs often flag values only below about 10 to 15 micrograms per litre, yet most authorities now treat ferritin under 30 as iron deficiency, and symptoms such as fatigue, hair shedding and restless legs commonly persist until ferritin is well above that floor. This guide explains why the reference range and the functional threshold are not the same number, how to read ferritin alongside transferrin saturation and CRP, and what the evidence says about treatment, including the case for alternate-day oral iron.
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Key facts
- Ferritin below 30 micrograms per litre is the threshold most UK and international bodies use for iron deficiency, well above the lower limit many lab printouts flag.12
- Ferritin is an acute-phase protein: infection or inflammation can push it up, so a CRP should be read alongside it. With inflammation present, deficiency is still likely below about 70.3
- You can be iron deficient without being anaemic. Fatigue, hair shedding, restless legs and breathlessness can all appear before haemoglobin falls.4
- Alternate-day single-dose oral iron absorbs a larger fraction than the same iron split across the day, because each dose raises hepcidin and blunts absorption for roughly 24 hours.56
Why a “normal” ferritin can still be too low
Ferritin is the body’s iron-storage protein, and the serum level tracks how much iron you have banked. The confusion starts because the “normal range” printed next to your result is a statistical range, not a health range. It is built from where most of a reference population sits, including plenty of people who are themselves quietly iron deplete. So the bottom of that range can be lower than the level at which the average person runs out of usable iron.
This is why someone can be told their bloods are “fine” with a ferritin of 18 or 22, yet still have empty iron stores. The honest interpretation is not “in range, therefore healthy” but “below the functional threshold, therefore deficient.” UK guidance has moved towards a single figure: a serum ferritin below 30 micrograms per litre reliably indicates iron deficiency in adults.12 The World Health Organization sets the diagnostic cut-off at 15 in apparently healthy people, but raises it to 70 in the presence of inflammation, which tells you the “real” number depends on context, not on the lab’s lower flag alone.3
Lab reference range vs functional threshold
It helps to separate three distinct numbers that often get conflated.
| Threshold type | Typical figure | What it means |
|---|---|---|
| Lab lower flag | ~10 to 15 | Where many UK lab reports start flagging “low”. Below this, deficiency is unarguable.3 |
| Diagnostic threshold (UK and clinical) | Below 30 | Iron deficiency in adults, used by NICE and the British Society of Gastroenterology.12 |
| Functional / symptomatic floor | ~30 to 50, higher in some conditions | Level below which iron-responsive symptoms often persist; restless legs guidance treats up to 75.7 |
| With inflammation (raised CRP) | Below 70 | Inflammation falsely lifts ferritin, so a higher cut-off is needed to avoid missing deficiency.3 |
The practical takeaway: a ferritin of 20 to 30 is not reassuring if you have classic symptoms, and a ferritin of 40 to 60 may still be too low if your CRP is up or if you have restless legs. The number has to be read against your symptoms and your inflammatory state, never in isolation. You can read more about interpreting common bloods on our health library.
Symptoms beyond anaemia
Most people think iron deficiency equals anaemia, but iron does far more than build haemoglobin. It is a cofactor for energy production in mitochondria, for dopamine signalling in the brain, and for the enzymes that run hair and nail growth. Iron stores fall in a sequence: first ferritin drops, then transferrin saturation falls, and only later does haemoglobin decline. That means you can have iron deficiency without anaemia and feel genuinely unwell while your full blood count still reads normal.4
- Fatigue and reduced exercise capacity. Randomised trials in non-anaemic, iron-deficient women have shown that iron supplementation reduces fatigue. One French trial of around 200 women found a meaningful drop in fatigue scores after oral iron in those with ferritin below 50.4
- Hair shedding (telogen effluvium). Low ferritin is consistently associated with diffuse hair loss in women, and many dermatologists aim to lift ferritin above 30 to 50 when treating it. The evidence is associative rather than from large randomised trials, so treat correction as supportive, not guaranteed.8
- Restless legs syndrome. Iron is central to the dopamine pathways involved. International guidelines recommend a trial of iron when ferritin is at or below 75 with transferrin saturation under 45 percent, a far higher target than for anaemia.7
- Breathlessness, palpitations and brain fog. These can appear as stores fall, partly through reduced oxygen-carrying reserve and partly through the metabolic roles of iron.
Evidence strength: the link between low ferritin and these symptoms is well established, but the size of the benefit from correcting it varies. Fatigue and restless legs have the strongest trial support; hair-loss benefit rests mainly on observational data. Correcting a genuinely low ferritin is low-risk and reasonable, but it is not a guaranteed cure for any single symptom.
What actually causes it
Iron deficiency is a sign, not a final diagnosis. The job is always to find the cause, because the same low ferritin can come from harmless or serious origins. The British Society of Gastroenterology stresses that confirmed iron deficiency anaemia, especially in men and postmenopausal women, warrants investigation for a gastrointestinal source before it is simply treated.2
- Blood loss. The commonest cause. Heavy menstrual bleeding in those who menstruate; gastrointestinal bleeding (ulcers, polyps, cancer, inflammatory bowel disease) in older adults and men.2
- Reduced absorption. Coeliac disease is a key reversible cause and should be screened for; H. pylori, atrophic gastritis, bariatric surgery and long-term proton-pump inhibitors also reduce iron uptake.2
- Increased demand. Pregnancy, growth in adolescence, and high-volume endurance training all raise requirements.
- Low intake. Vegetarian and vegan diets supply non-haem iron, which is absorbed less efficiently, so deficiency is more common without attention to intake.
Reading ferritin, transferrin saturation and CRP together
A single ferritin is useful but can mislead, because ferritin rises with inflammation, liver disease, alcohol and infection. The fix is to read it as a small panel rather than a solo number.
Ferritin estimates your iron stores. Transferrin saturation (serum iron divided by total iron-binding capacity) estimates how much iron is actually circulating and available; a value under about 20 percent supports deficiency. CRP is the referee: if it is raised, ferritin may be falsely reassuring, so you lean on a higher ferritin cut-off and on transferrin saturation instead.3
| Ferritin | TSAT | CRP | Likely picture |
|---|---|---|---|
| Below 30 | Low (under ~20%) | Normal | Straightforward iron deficiency.1 |
| 30 to 100 | Low (under ~20%) | Raised | Likely deficiency masked by inflammation; treat and find the inflammatory cause.3 |
| Normal or high | Normal or high | Raised | Anaemia of inflammation rather than simple deficiency. |
| 30 to 50 | Borderline | Normal | Possible functional shortfall; judge against symptoms (fatigue, restless legs, hair loss).7 |
This is exactly the kind of pattern reading that turns a confusing “normal” result into a clear answer. If you are trying to make sense of your own numbers, our insights articles cover how to approach lab interpretation more broadly.
Treatment: dose, route and timing
For most people, treatment is oral iron, and the UK approach is simpler than the old advice. NICE recommends a single daily tablet of ferrous sulfate, ferrous fumarate or ferrous gluconate, each providing roughly 40 to 65 milligrams of elemental iron, rather than the older two- or three-times-daily regimens.1 A ferrous sulfate 200 milligram tablet supplies about 65 milligrams of elemental iron; ferrous fumarate 210 milligrams supplies a similar amount; ferrous gluconate is lower per tablet.
The alternate-day dosing evidence
The most important recent shift comes from the physiology of hepcidin, the hormone that controls iron absorption. When you take an iron dose, hepcidin rises and stays elevated for around 24 hours, which means a second dose taken the same day, or even the next morning, is absorbed less well. Two careful studies from the same group established this.
Moretti and colleagues, publishing in Blood in 2015, gave iron-depleted young women labelled iron and showed that doses of 60 milligrams or more raised hepcidin for up to 24 hours and reduced absorption from later doses; twice-daily dosing cut absorption from the second dose by roughly a third.5 Building on this, Stoffel and colleagues ran randomised trials (Lancet Haematology, 2017) in iron-depleted women and found that giving iron on alternate days as a single morning dose produced higher fractional absorption (about 22 percent vs 16 percent) and greater total absorption than consecutive-day dosing.6 A follow-up in iron-deficient anaemic women (Haematologica, 2020) confirmed the same advantage in people who were actually anaemic.9
What this does and does not show: these trials measured iron absorption robustly. They were short and did not prove that alternate-day dosing corrects anaemia faster over months. A reasonable reading, and increasingly common UK practice, is that alternate-day single dosing absorbs efficiently and is often better tolerated, which can improve adherence; daily dosing remains acceptable, especially when speed matters.
Practical points that genuinely help absorption: take iron on an empty stomach if you can tolerate it, with a source of vitamin C, and separate it from tea, coffee, calcium and antacids. Gut side effects (nausea, constipation, dark stools) are common; alternate-day dosing or a lower elemental dose often eases them without losing much benefit.
When intravenous iron is used
Intravenous iron is not first-line for most people, but it is the right choice in defined situations: intolerance or non-response to oral iron, ongoing losses that outpace oral replacement, chronic kidney disease, inflammatory bowel disease, after bariatric surgery, and where rapid repletion is needed (for example later pregnancy or before surgery).210 Modern preparations such as ferric carboxymaltose and ferric derisomaltose allow large doses in one or two visits.
Safety note: ferric carboxymaltose can cause hypophosphataemia (low blood phosphate), which is sometimes prolonged and can cause fatigue, weakness or bone pain. This is more likely with repeated dosing and severe deficiency. If those symptoms appear after an infusion, phosphate should be checked.11 All IV iron carries a small risk of infusion reactions and must be given where these can be managed. This article is general information, not a treatment recommendation.
Monitoring and when to refer
With oral iron, NICE advises rechecking the full blood count after about 2 to 4 weeks; a haemoglobin rise of around 20 grams per litre over roughly four weeks confirms a good response. Treatment should continue until haemoglobin normalises and then for about three further months to refill stores, with periodic checks afterwards.1 If there is no response, the usual culprits are poor adherence, continued blood loss, malabsorption (think coeliac disease) or the wrong diagnosis.
Referral is warranted when the cause is unclear, when there are red-flag features (gastrointestinal bleeding, weight loss, a strong family history of bowel cancer), in iron deficiency anaemia in men or postmenopausal women without an obvious source, and when oral iron repeatedly fails. The investigation pathway, including coeliac screening and endoscopy where appropriate, follows the British Society of Gastroenterology guidance.2 If you are weighing up supplements alongside treatment, our stack builder can help you organise what you are taking.
What to ask your GP
- My ferritin is in range but I have fatigue, hair shedding or restless legs: can we treat it as functional iron deficiency rather than wait for anaemia?
- Can we check transferrin saturation and CRP alongside ferritin, so a normal ferritin isn’t masking deficiency hidden by inflammation?
- Would alternate-day single-dose oral iron be reasonable for me, given the absorption and tolerability evidence?
- Do I need investigation for a cause (heavy periods, coeliac screen, gut bleeding) rather than iron alone, especially if I am male or postmenopausal?
- When should we recheck my bloods, and how long should I keep taking iron after my levels normalise?
References
- NICE Clinical Knowledge Summaries. Anaemia - iron deficiency: diagnosis and management. cks.nice.org.uk, accessed 2026.
- Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. 2021;70:2030-2051. BSG guideline (PDF).
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. 2020. who.int.
- Vaucher P, et al. Effect of iron supplementation on fatigue in non-anaemic menstruating women with low ferritin: a randomised controlled trial. CMAJ. 2012;184(11):1247-1254. PMC3414597.
- Moretti D, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981-1989. ashpublications.org.
- Stoffel NU, et al. 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. Lancet Haematol. 2017;4(11):e524-e533. thelancet.com.
- Allen RP, et al. 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. Sleep Med. 2018;41:27-44. PMID 29425576.
- Trost LB, Bergfeld WF, Calogeras E. The diagnosis and treatment of iron deficiency and its potential relationship to hair loss. J Am Acad Dermatol. 2006;54(5):824-844. PMID 16635664.
- Stoffel NU, et al. Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica. 2020;105(5):1232-1239. PMID 31413088.
- Pavord S, et al. UK guidelines on the management of iron deficiency in pregnancy (British Society for Haematology). Br J Haematol. 2020;188(6):819-830. onlinelibrary.wiley.com.
- Schaefer B, et al. Hypophosphataemia after treatment of iron deficiency with intravenous ferric carboxymaltose or iron isomaltoside: a systematic review and meta-analysis. Br J Clin Pharmacol. 2021;87(5):2256-2273. onlinelibrary.wiley.com.
This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.