You feel exhausted for weeks. You go to the doctor, get a blood test, and hear that your haemoglobin is normal, so you’re not anaemic. Case closed. Except you’re still tired.
For a lot of people, especially women who menstruate, the missing piece is ferritin: the measure of how much iron your body has in storage. Your iron reserves can run low long before your haemoglobin drops, and research suggests that low stores alone can be enough to cause fatigue. In September 2026, the American Society of Hematology (ASH) went further and raised the ferritin level at which iron deficiency should be diagnosed, so many people who were told their result was “fine” would now meet the definition.
This article is part of our complete guide on how to increase your energy levels naturally. It covers what ferritin is, what the trials show about low iron without anaemia, the new diagnostic cut-offs, and how to get tested safely. It is general information, not medical advice. Please don’t start iron supplements without a blood test, because too much iron is harmful.
Ferritin vs Haemoglobin: Two Different Questions
A standard full blood count measures haemoglobin, the iron-containing protein in red blood cells that carries oxygen. If it’s low, you have anaemia. Ferritin is a different test. It reflects the iron your body has stored away, mostly in the liver, to call on when it needs it.
The body protects haemoglobin first. When iron intake doesn’t keep up with losses, it draws down its stores to keep red blood cell production going. So iron deficiency develops in stages:
- Depleted stores: ferritin falls, haemoglobin is still normal.
- Iron-deficient red cell production: stores are nearly empty, and supply to new red cells starts to suffer.
- Iron deficiency anaemia: haemoglobin finally drops below the normal range.
If your doctor only checks haemoglobin, stages 1 and 2 are invisible. Iron isn’t only used for red blood cells, either. It’s also needed for energy production in your cells, which is one reason low stores may cause symptoms before anaemia appears.
We’ve covered one well-known example of this before: how low ferritin can cause hair fall. This article looks at the more general symptom, fatigue.
What the Trials Show About Iron and Fatigue Without Anaemia
The obvious question is whether low ferritin actually causes tiredness, or whether it’s just a coincidence in tired people. The best way to find out is a randomised trial: give iron or a placebo to fatigued people who aren’t anaemic, and see what happens. Several such trials exist.
The 2003 BMJ trial
In a double-blind trial published in the BMJ (Verdon et al., 2003), 144 women aged 18 to 55 with unexplained fatigue and normal haemoglobin took either 80 mg of elemental iron a day or a placebo for four weeks. Fatigue fell by 29% in the iron group and 13% in the placebo group. About half of the women had a ferritin of 20 μg/L or lower at the start. The authors concluded that the benefit is likely limited to women with low or borderline ferritin.
The 2012 CMAJ trial
A larger trial in the Canadian Medical Association Journal (Vaucher et al., 2012) recruited 198 menstruating women aged 18 to 53 from primary care practices in France. All of them were tired, had a ferritin below 50 μg/L, and had haemoglobin above 12 g/dL, so none were anaemic. They took 80 mg of elemental iron a day or a placebo for 12 weeks.
Fatigue scores fell by 47.7% with iron and 28.8% with placebo. That’s a real difference over and above the placebo effect, which was large too. Iron did not improve quality of life, depression or anxiety scores. The authors suggested that women with unexplained fatigue and ferritin below 50 μg/L should be considered for iron supplementation.
What the evidence adds up to
A 2018 systematic review in BMJ Open (Houston et al.) pooled the randomised trials in iron-deficient adults without anaemia. Iron reduced people’s own ratings of fatigue. It did not improve objective measures of physical capacity, such as exercise tests.
So the honest summary is this. If you’re tired, not anaemic, and your ferritin is low, correcting it has a reasonable chance of helping you feel less fatigued. The effect is modest, though, and iron isn’t a cure-all. Many people in these trials were still somewhat tired afterwards, because fatigue usually has more than one cause.
What Counts as “Low” Ferritin? The 2026 Change
This is where many people fall through the cracks. For decades, a common cut-off for iron deficiency in adults was a ferritin below 15 μg/L. (μg/L and ng/mL are the same thing, so 15 μg/L = 15 ng/mL.) The World Health Organization kept that threshold in its 2020 guideline, while noting that it was based on expert opinion rather than strong data.
A threshold that low misses a lot of genuine iron deficiency. In September 2026, ASH published new guidance in Blood Advances on how to diagnose iron deficiency. As reported in coverage of the guideline, the main changes for adults are:
- A ferritin of 30 ng/mL or lower is the suggested threshold for diagnosing iron deficiency in adults, including people who menstruate and pregnant people. ASH recommends against using the old 15 ng/mL cut-off.
- A threshold of up to 50 ng/mL may be appropriate for people with symptoms consistent with iron deficiency or with risk factors such as heavy menstrual bleeding, upcoming surgery or plans for pregnancy.
- Inflammation can hide iron deficiency. Ferritin rises during infection and inflammation, so a “normal” result can be misleading. For adults with inflammation, ASH suggests interpreting ferritin together with transferrin saturation, another iron blood test, rather than relying on ferritin alone.
The evidence behind these recommendations is mostly rated low certainty, and they’re conditional rather than strong. Still, the direction is clear: a ferritin of 20 or 25 ng/mL, which many labs would have flagged as normal, now meets the definition of iron deficiency for most adults. If you were told your ferritin was “fine” a while ago, it’s worth looking up the actual number.
Who Is Most at Risk of Low Iron?
Iron needs are not the same for everyone. According to the US National Institutes of Health Office of Dietary Supplements, the recommended daily intake is 18 mg for women aged 19 to 50, but only 8 mg for men and for adults over 50. That difference is mainly due to menstrual blood loss. Groups commonly at higher risk include:
- People with heavy or long periods
- Pregnant people (the recommended intake rises to 27 mg a day)
- Frequent blood donors
- Vegetarians and vegans, because iron from plant foods is absorbed less well than iron from meat and fish
- Endurance athletes
- People with conditions that affect absorption, such as coeliac disease or inflammatory bowel disease, or who have had weight-loss surgery
- Anyone with ongoing blood loss from the gut, which needs medical investigation
That last point matters. In men and in people who no longer menstruate, low iron with no obvious reason should be investigated by a doctor, because it can be the first sign of bleeding somewhere in the digestive tract.
How to Get Tested
If you’ve been tired for a few weeks without an obvious reason, the NHS advises seeing a GP, who may arrange blood tests. When you go, it’s reasonable to ask specifically whether ferritin is being checked, not just a full blood count. If you’ve had a result before, ask for the actual number rather than “normal” or “abnormal.”
Mention any recent infection or illness, since inflammation can push ferritin up temporarily, and your doctor may want to check transferrin saturation or a marker of inflammation (such as CRP) as well. Fatigue has many other possible causes too, including thyroid problems, diabetes, poor sleep, depression and sleep apnoea. So a low ferritin doesn’t rule out something else going on. For more on tiredness that persists despite enough sleep, see why you feel tired all the time even after sleeping 8 hours.
If Your Ferritin Is Low: Food, Supplements and Safety
Food first, but know its limits
Iron in food comes in two forms. Haem iron, from meat, fish and poultry, is absorbed relatively well. Non-haem iron, from beans, lentils, tofu, leafy greens, fortified cereals and seeds, is absorbed less well. Eating a source of vitamin C (citrus, peppers, tomatoes) with plant-based iron improves absorption, while tea and coffee taken with a meal reduce it. Diet alone helps maintain iron levels, but it’s often slow to correct stores that are already low.
Supplements: dose and timing
Your doctor will advise on dose and duration. The trials above used 80 mg of elemental iron a day. One notable finding relates to timing. In two small trials in iron-depleted women published in The Lancet Haematology in 2017 (Stoffel et al.), taking iron on alternate days led to greater total absorption than taking it every day. Daily doses raise hepcidin, a hormone that blocks iron absorption for a day or so afterwards. It’s still unclear whether that makes a real difference to how quickly ferritin recovers, so don’t change a prescribed regimen without asking. Constipation, nausea and stomach upset are common side effects, and alternate-day dosing is something worth discussing with your doctor if they bother you.
Why you shouldn’t supplement blind
Iron is one of the few supplements where more is clearly not better. The tolerable upper intake level for adults is 45 mg a day, set mainly because of gastrointestinal side effects. A doctor may prescribe more than that for a diagnosed deficiency, but that’s done with monitoring. Some people have hereditary haemochromatosis, a common genetic condition in which the body absorbs too much iron, and supplements can cause serious harm in them. Iron is also a leading cause of poisoning in young children, so keep tablets out of their reach. Test first, treat if needed, and re-test to check that it worked.
The Bottom Line
Normal haemoglobin doesn’t mean normal iron. If you’re tired for weeks without a clear reason, especially if you menstruate, ask your doctor to check your ferritin, and ask for the number. Under the 2026 ASH guidance, a ferritin of 30 ng/mL or below counts as iron deficiency in most adults, and up to 50 ng/mL may count if you have symptoms or risk factors. Correcting low iron can modestly reduce fatigue, but it’s one piece of the energy picture alongside sleep, movement, hydration and stress. Our complete guide to increasing your energy levels naturally covers the rest.
Frequently Asked Questions
Can you have low ferritin without being anaemic?
Yes, and it’s common. Ferritin reflects stored iron, which runs down before haemoglobin falls. Many people, especially women who menstruate, have low ferritin with haemoglobin still in the normal range.
What ferritin level causes fatigue?
There’s no single number. In a 2012 randomised trial, iron reduced fatigue in non-anaemic women with ferritin below 50 μg/L. The 2026 ASH guideline suggests diagnosing iron deficiency at 30 ng/mL or lower in adults, or up to 50 ng/mL with symptoms or risk factors.
How long does it take for iron supplements to help with tiredness?
In the trials, fatigue improved over 4 to 12 weeks. Refilling iron stores usually takes longer, which is why doctors typically re-check ferritin after a few months of treatment.
Should I take iron supplements if I’m always tired?
Not without a blood test. Iron only helps if you’re actually low, and excess iron can be harmful, particularly for people with haemochromatosis. Ask your doctor to check ferritin and other common causes of fatigue first.
Can high ferritin also cause fatigue?
High ferritin can be a sign of iron overload or of inflammation, liver disease and other conditions, some of which can cause tiredness. A high result needs follow-up with a doctor rather than any change in diet or supplements on your own.
