Iron deficiency is one of the most common nutritional deficiencies worldwide — and one of the most frequently missed in primary care. People with iron deficiency often spend months feeling exhausted, breathless, or unwell before anyone checks their ferritin. Yet the tests are simple, cheap, and highly informative if you know how to read them. Here's what each result actually means.
Iron Deficiency vs. Anaemia — What's the Difference?
These two terms are often used interchangeably, but they describe different stages of the same problem. Iron deficiency means your body's iron stores are low — but you may not yet have anaemia. Iron deficiency anaemia means the deficiency has progressed to the point where your red blood cell count and haemoglobin have also fallen.
It's entirely possible to have depleted iron stores (low ferritin) with a normal haemoglobin and still feel terrible. This is called "iron deficiency without anaemia" — and it's underdiagnosed because many GPs only check haemoglobin rather than ferritin.
The Key Tests Explained
Ferritin — Your Iron Store
Ferritin is a protein that stores iron in your cells. Measuring ferritin gives us a direct window into your body's iron reserves. It's the most sensitive single marker for iron deficiency — your ferritin will fall long before your haemoglobin does. A low ferritin is the earliest sign of iron depletion and is enough to explain fatigue, hair loss, and poor concentration even when everything else looks normal.
The catch: ferritin is an acute phase reactant. This means it rises in response to inflammation, infection, or liver disease — even if your iron stores are actually low. In other words, a "normal" or high ferritin in someone with active inflammation doesn't rule out iron deficiency. Your GP may order additional tests (serum iron, transferrin saturation) in this situation.
Haemoglobin (Hb) — Oxygen-Carrying Capacity
Haemoglobin is the iron-containing protein in red blood cells that carries oxygen. When it falls below the reference range, this is classified as anaemia. Haemoglobin is measured as part of your Full Blood Count (FBC).
MCV — Mean Corpuscular Volume
MCV tells us the average size of your red blood cells. In iron deficiency anaemia, red cells become smaller than normal (microcytic, MCV below 80 fL) because they don't have enough iron to fill out properly. This is a useful clue to the type of anaemia.
By contrast, anaemia caused by B12 or folate deficiency produces macrocytic red cells (MCV above 100 fL) — large, misshapen cells. This distinction changes the treatment entirely.
Reference Ranges at a Glance
| Test | Normal Range | Low / Deficient |
|---|---|---|
| Ferritin | 15–300 µg/L (men) · 15–200 µg/L (women) | Below 15 µg/L (symptomatic below 30 µg/L in many guidelines) |
| Haemoglobin (men) | 130–170 g/L | Below 130 g/L = anaemia |
| Haemoglobin (women) | 120–160 g/L | Below 120 g/L = anaemia |
| MCV | 80–100 fL | Below 80 fL = microcytic (suggests iron deficiency) |
| Serum Iron | 10–30 µmol/L | Below 10 µmol/L = depleted |
| Transferrin Saturation | 20–45% | Below 16% = functional iron deficiency |
The NHS laboratory "normal" range for ferritin typically starts at 15 µg/L. However, many clinicians — and the British Society for Haematology — recognise that people can be symptomatic with iron deficiency at levels up to 30 µg/L. If your ferritin is between 15–30 µg/L and you have classic symptoms, it's worth discussing a trial of iron supplementation with your GP.
What Causes Iron Deficiency?
Iron deficiency always has a cause. The most common are: blood loss (heavy periods in women; gastrointestinal bleeding in men and postmenopausal women — this should always be investigated), poor dietary intake (particularly in vegans and vegetarians, since plant-based iron is less bioavailable), malabsorption (coeliac disease, post-bariatric surgery, inflammatory bowel disease), and increased demand (pregnancy, breastfeeding, rapid growth in adolescents).
In men and postmenopausal women with iron deficiency anaemia and no obvious dietary cause, the NHS recommends investigation to exclude gastrointestinal blood loss — including a stool test for occult blood and often a bowel investigation. This is because bowel cancer and gastric cancer can cause iron deficiency through slow, invisible bleeding.
Symptoms of Iron Deficiency
The classic symptoms are: persistent fatigue disproportionate to sleep, breathlessness on exertion, palpitations, pallor (particularly noticeable on the inner eyelids and nail beds), headaches, difficulty concentrating ("brain fog"), cold intolerance, restless legs at night, brittle nails, and hair loss. In severe deficiency, people may develop pica — unusual cravings for non-food items like ice, clay, or chalk.
Not everyone has all of these. Some people with significantly low ferritin feel "fine" on the surface but describe a subjective sense that something isn't right — a vague lack of vitality that they've normalised over time.
Treatment — Iron Supplements
The standard first-line treatment is oral iron supplementation — typically ferrous sulfate 200mg twice daily (providing approximately 65mg of elemental iron per dose). It's taken on an empty stomach for best absorption, but can be taken with food if it causes significant GI upset.
Common side effects include constipation, nausea, dark stools, and stomach cramps. If these are intolerable, alternatives include ferrous fumarate, ferrous gluconate, or liquid iron preparations. Vitamin C taken alongside iron supplements significantly improves absorption. Avoid taking iron within 2 hours of dairy products, tea, coffee, or certain medications (particularly levothyroxine).
Replenishing iron stores takes time — haemoglobin should start to rise within 2–4 weeks of starting treatment, but ferritin takes much longer to rebuild. Most guidelines recommend continuing iron for at least 3 months after haemoglobin normalises to ensure stores are fully replenished. A repeat blood test at 3 months is standard.
If oral iron is poorly tolerated, not absorbed adequately, or if the deficiency is severe, intravenous iron infusion is increasingly available in the NHS — ask your GP for a referral to a haematology or gastroenterology clinic if oral treatment isn't working.
⚕️ Medical disclaimer: Iron deficiency always needs investigation to find the underlying cause. Do not self-supplement without a blood test confirming deficiency — iron overload is also harmful. Speak with your GP before starting iron therapy.
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