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 over 37 years.
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 conversion, 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.
The functional optimal of 50–100 µg/L for women (70–150 for men) reflects the level at which iron-dependent physiology actually functions well — not merely the level at which you avoid a diagnosis.
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. Ferritin below 50 µg/L is consistently associated with diffuse hair thinning — 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 of the most clinically important consequences of low ferritin that rarely gets mentioned: iron is a required cofactor for the thyroid peroxidase (TPO) enzyme — the enzyme responsible for producing thyroid hormone. Iron is also required for the deiodinase enzymes that convert inactive T4 into active Free T3.
This means that low ferritin directly impairs thyroid function — specifically the conversion of T4 to T3 — even when thyroid markers appear acceptable on standard testing. I have seen clients with normal TSH, normal Free T4, and persistently low Free T3 whose thyroid numbers improved meaningfully once ferritin was brought up to the functional optimal. The thyroid wasn't the primary problem. The iron was.
"In 37 years of clinical practice, 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 50 µg/L, iron repletion is the clinical priority regardless of haemoglobin. Ferrous bisglycinate is the best-tolerated supplemental form. Avoid tea and coffee within an hour of iron supplementation — tannins reduce absorption by up to 80%. Allow 3–6 months to reach the functional optimal and retest.
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.