Every week, someone sits in a GP surgery, exhausted, losing hair, breathless climbing stairs, and unable to concentrate — and is told their blood results are normal. The haemoglobin is above 120. Not anaemic. Off you go.
What the GP didn't check — or didn't flag if they did — was ferritin. And if they did check it, the number sitting at 22 or 28 or 31 was waved through as acceptable. The reference range says anything above 15 is normal. The client is not anaemic. The consultation ends.
This is the most commonly missed iron pattern in functional medicine. It has a name: iron deficiency without anaemia. And it explains a significant proportion of the "unexplained" fatigue cases I've seen across nearly forty years.
What does my ferritin number mean?
Ferritin in µg/L is the same number as in ng/mL.
| Ferritin | What it suggests | Source |
|---|---|---|
| below 15 | Iron stores absent: iron deficiency is near-certain. | BSG 2021; WHO for healthy women (Mei 2023) |
| 15–29 | Low iron stores: iron deficiency likely. In people seeing a doctor, a ferritin below 30 picked up iron deficiency well (sensitivity 79%, specificity 98%; low-certainty evidence). | BSG 2021; Garcia-Casal 2021 (Cochrane) |
| 30–44 | A grey zone: the BSG suggests 45 as the best balance for spotting iron deficiency in practice. | BSG 2021 |
| 45–99 | Usually enough stored iron. My working target is 50–100 (a subgroup in one trial: fatigue improved with iron only at 50 or below). | Verdon 2003; my practice |
| 100–150 | Above my target: no reason to take iron on this number alone. | My practice |
| above 150 | Iron deficiency unlikely, even with inflammation; a high ferritin has other causes worth checking with your GP. | BSG 2021 |
Ferritin rises with inflammation or infection, so a ‘normal’ number can hide low iron. Check it with CRP; WHO uses below 70 µg/L (70 ng/mL) in adults with inflammation (WHO 2020), and the BSG notes up to 100 µg/L (100 ng/mL) can still mean iron deficiency in active inflammatory bowel disease. A ferritin on its own doesn’t explain tiredness: your GP should also look at why it’s low.
Grade: guideline cut-offs (BSG 2021; WHO); my target is labelled as mine. The same bands are on Ferritin: normal or optimal?
What ferritin actually is
Ferritin is the body's iron storage protein. Think of it as the warehouse — the reserve supply the body draws on when circulating iron is insufficient to meet demand. Haemoglobin, by contrast, is the delivery driver. The NHS checks the delivery driver. It doesn't check the warehouse.
The critical clinical point is this: haemoglobin only falls once iron stores are severely depleted. By the time haemoglobin drops below the anaemia threshold, the ferritin has typically been low for months — sometimes years. The warehouse was empty long before the delivery driver slowed down.
Meanwhile, every iron-dependent process in the body has been running at reduced capacity: energy production, thyroid hormone production, hair follicle cycling, immune function, cognitive performance, temperature regulation. All of these require iron. None of them require you to be technically anaemic before they start to suffer.
The reference range problem
The conventional lower reference of 15 µg/L was derived statistically — it represents the bottom two percent of the tested population. But that population is not healthy. It's a population that includes people who are already unwell, nutritionally depleted, and iron deficient. Deriving a normal range from an unwell population produces an unwell normal range.
My clinical target of 50–100 µg/L, for women and men, is a target, not a proven optimum. The lower end rests on the one trial that tested it: in non-anaemic women with unexplained fatigue, iron improved fatigue only when ferritin was 50 µg/L or below (Verdon, BMJ 2003). There is no evidence of benefit from pushing higher, and some evidence of risk.
What symptoms look like below 50 µg/L
Below the functional optimal, iron-dependent processes begin to underperform. The symptoms are not dramatic — they creep in gradually, which is part of why they're so often dismissed.
Hair loss deserves particular mention. The hair follicle is one of the most metabolically active structures in the body. It requires iron continuously. Lower ferritin is associated with diffuse hair thinning in women, though no specific threshold is validated — the kind that appears as general volume loss rather than distinct bald patches. This is reversible with iron repletion, but the hair cycle means you typically need 3–6 months of sustained repletion before you see the improvement.
The thyroid connection
One consequence of low iron that rarely gets mentioned: thyroid peroxidase (TPO), the enzyme that produces thyroid hormone, is an iron-containing (haem) enzyme. The enzymes that convert T4 into active T3 are selenium enzymes rather than iron enzymes.
In observational studies, iron-deficiency anaemia is associated with lower T4 and T3. I have seen clients whose thyroid numbers improved once iron was repleted — that is clinical experience, not trial evidence, and it is a reason to check iron before assuming the thyroid is the primary problem.
"In nearly forty years of working with people, ferritin below 50 µg/L is one of the most consistent findings in women presenting with fatigue, hair loss, and low mood — and the one most reliably dismissed by standard investigation."
Why the standard test misses it
Standard NHS iron assessment typically runs haemoglobin and occasionally serum iron. Ferritin is not always included. When it is included and returns at 22 µg/L, the lab report shows it within the reference range and the GP has no clinical reason to act.
This is not a failure of the GP — it's a failure of the reference range. The GP is doing exactly what the system asks them to do: identify disease. Iron deficiency without anaemia is not classified as disease. It is the territory between optimal and diagnosed — which is precisely where functional medicine operates.
What to do if you suspect this pattern
Ask for ferritin specifically. If you're having private blood testing, include it alongside serum iron, TIBC, and haemoglobin — the full iron panel gives a much more complete picture than ferritin alone. Transferrin saturation (serum iron divided by TIBC, multiplied by 100) adds important context: below 16% confirms iron is not reaching tissues even if ferritin appears borderline.
If ferritin is below 30 µg/L (30 ng/mL), your iron stores are low whatever your haemoglobin (BSG 2021), and the first job is finding out why, with your GP. The BSG notes that whether iron helps people with low ferritin but no anaemia is unclear. Ferrous bisglycinate is the form I usually use. In iron-depleted women, a single morning dose on alternate days was absorbed better than daily dosing (Stoffel 2017), and tea and coffee with a meal reduce absorption. Doses are set individually after testing, with your other medicines checked. Retest to confirm the change rather than assuming it.
In women with heavy menstrual bleeding, addressing the blood loss is as important as supplementing iron — you can't fill a bucket with a hole in it. In men and post-menopausal women with unexplained low ferritin, the cause warrants investigation rather than just supplementation.
The gap between ferritin 22 and ferritin 75 is the gap between feeling inexplicably terrible and beginning to understand why. It is one of the most addressable findings in functional medicine. It is also one of the most reliably missed.
References
- Garcia-Casal MN et al. Serum or plasma ferritin concentration as an index of iron deficiency and overload. Cochrane Database Syst Rev 2021;5:CD011817. PMID 34028001. doi:10.1002/14651858.CD011817.pub2. In adults seeing a doctor, ferritin below 30 µg/L: sensitivity 79%, specificity 98% (low-certainty evidence).
- Mei Z et al. Comparison of current World Health Organization guidelines with physiologically based serum ferritin thresholds for iron deficiency in healthy young children and nonpregnant women using data from the Third National Health and Nutrition Examination Survey. J Nutr 2023;153:771–80. PMID 36803577. doi:10.1016/j.tjnut.2023.01.035.
- Snook J et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut 2021;70(11):2030–51. PMID 34497146. doi:10.1136/gutjnl-2021-325210. Below 15 µg/L: absent iron stores; below 30 µg/L: low iron stores; up to 100 µg/L can still mean iron deficiency in active inflammatory bowel disease.
- 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:e524–33. PMID 29032957. doi:10.1016/S2352-3026(17)30182-5
- Verdon F et al. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial. BMJ 2003;326:1124. PMID 12763985. doi:10.1136/bmj.326.7399.1124
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020. ISBN 9789240000124. NBK569877