NUTRITION & DEFICIENCY
Iron Deficiency Blood Test UK: What's Tested, What Results Mean & When to Worry
Iron deficiency is the most common nutritional deficiency in the world — and the UK is no exception. The NHS estimates that around 3 million people in the UK have iron deficiency anaemia, with many more in the earlier stages of depletion where symptoms are already present but haemoglobin hasn't dropped below the clinical threshold.
The problem? A standard GP blood test often only checks ferritin — and sometimes not even that. If your ferritin comes back "normal" at 15 µg/L, you'll be told you're fine. But research increasingly shows that ferritin below 30 µg/L is associated with fatigue, hair loss, and impaired exercise performance, even without anaemia.
This guide explains every marker in a comprehensive iron panel, what the results actually mean, when the NHS will (and won't) test you, and why a full iron study reveals far more than ferritin alone.
1. What Is Iron Deficiency?
Iron is an essential mineral that your body uses for oxygen transport (it's the core of haemoglobin in red blood cells), energy production (it drives the electron transport chain in mitochondria), immune function, and cognitive performance. You can't make iron — you have to absorb it from food or supplements.
Iron deficiency means your body's iron stores have dropped below the level needed to maintain normal function. It's not the same as anaemia — anaemia is the final stage, when haemoglobin production is measurably impaired. Most people experience symptoms long before that point.
The World Health Organisation calls iron deficiency the most prevalent nutritional disorder globally, affecting an estimated 2 billion people. In the UK, the National Diet and Nutrition Survey (NDNS) consistently finds that 27% of women aged 19–64 have iron intakes below the Lower Reference Nutrient Intake — the level below which deficiency is almost certain.
2. The Three Stages: Depletion → Deficiency → Anaemia
Iron deficiency doesn't happen overnight. It progresses through three clinically distinct stages, each with different blood test findings:
| STAGE | WHAT'S HAPPENING | KEY MARKER | SYMPTOMS? |
|---|---|---|---|
| 1. Iron depletion | Stores running low; supply still adequate | Ferritin ↓ (15–30 µg/L) | Often yes — fatigue, poor focus, hair thinning |
| 2. Iron-deficient erythropoiesis | Not enough iron reaching bone marrow for new red cells | Transferrin saturation ↓ (<16%) | Yes — above plus breathlessness on exertion |
| 3. Iron deficiency anaemia | Haemoglobin drops below clinical threshold | Haemoglobin ↓ (<120 g/L women, <130 g/L men) | Pronounced — pallor, palpitations, dizziness, severe fatigue |
The critical insight: the NICE guideline NG24 on blood transfusion and the British Society for Haematology (BSH) guidelines both recognise that symptoms begin in Stage 1, long before anaemia develops. Yet most GP practices only investigate when haemoglobin is abnormal — Stage 3.
3. Symptoms Most People Miss
Iron deficiency symptoms are notoriously non-specific — they overlap with thyroid disorders, vitamin D deficiency, depression, and simple exhaustion. That's why blood testing matters. Common signs include:
The BMJ notes that fatigue and cognitive impairment can occur at ferritin levels well above the anaemia threshold, particularly in premenopausal women and endurance athletes. If you recognise three or more symptoms above, a blood test is the only way to confirm whether iron is the cause.
4. Who Is Most at Risk in the UK?
Some groups are at significantly higher risk of iron deficiency due to increased losses, reduced intake, or impaired absorption:
Premenopausal women
Menstrual blood loss is the single biggest driver of iron deficiency in the UK. Women with heavy periods (menorrhagia) lose 5–6 times more iron per cycle than the average.
Pregnant women
Iron requirements roughly double during pregnancy. NICE recommends screening at booking and 28 weeks (NICE NG201).
Vegetarians and vegans
Non-haem iron (from plants) has 2–20% absorption vs 15–35% for haem iron (from meat). The NDNS shows vegans have the lowest mean ferritin across all dietary groups.
Endurance athletes
Runners, cyclists, and swimmers lose iron through sweat, GI micro-bleeding, and foot-strike haemolysis. Studies suggest up to 50% of female endurance athletes are iron deficient.
People with GI conditions
Coeliac disease, Crohn's disease, ulcerative colitis, and H. pylori infection all reduce iron absorption. NICE recommends checking ferritin in anyone with unexplained GI symptoms.
Regular blood donors
Each 470 mL donation removes approximately 250 mg of iron. NHS Blood and Transplant now tests ferritin and defers donors with low levels.
Older adults on proton pump inhibitors (PPIs)
Long-term PPI use reduces stomach acid, which is needed to convert dietary iron into its absorbable form.
5. What an Iron Blood Test Actually Measures
A comprehensive iron study includes several markers, each telling a different part of the story:
Ferritin →
The storage protein for iron. Think of it as your iron savings account. Low ferritin is the earliest and most sensitive marker of iron depletion. However, ferritin is also an acute-phase reactant — it rises during inflammation, infection, and liver disease, which can mask true deficiency.
Also included in: Essential panel · Nutrition panel
Serum iron
The amount of iron circulating in your blood right now, bound to transferrin. Fluctuates throughout the day (highest in the morning) and after meals, so it's a snapshot, not a trend. Best interpreted alongside TIBC and transferrin saturation.
Total Iron Binding Capacity (TIBC)
Measures how much iron your transferrin proteins could carry if fully loaded. When iron stores are low, your body makes more transferrin to try to capture every available iron atom — so TIBC goes up in deficiency. High TIBC + low ferritin is the classic iron deficiency pattern.
Transferrin saturation (TSAT)
Calculated as (serum iron ÷ TIBC) × 100. Tells you what percentage of your iron-carrying capacity is actually being used. Below 16% suggests iron-deficient erythropoiesis — your bone marrow isn't getting enough iron to make red blood cells efficiently. The BSH considers TSAT <16% diagnostic for functional iron deficiency.
Haemoglobin (Hb) →
The oxygen-carrying protein in red blood cells. Low haemoglobin confirms anaemia — but it's a late marker. By the time haemoglobin drops, you've been iron depleted for weeks or months. The WHO defines anaemia as Hb <120 g/L in women and <130 g/L in men.
Full blood count (FBC)
Often ordered alongside iron studies. Includes MCV (mean cell volume) — in iron deficiency, red blood cells become smaller than normal (microcytic, MCV <80 fL) and paler (hypochromic). However, MCV can be normal in early deficiency and misleading in combined deficiencies (e.g., concurrent B12 or folate deficiency raises MCV, masking the iron effect).
6. NHS Reference Ranges vs Optimal Levels
NHS "normal" ranges are designed to detect disease, not optimise health. The gap between "not anaemic" and "feeling your best" is significant:
| MARKER | NHS RANGE | OPTIMAL RANGE | SOURCE |
|---|---|---|---|
| Ferritin | 15–300 µg/L (men) 15–200 µg/L (women) | 50–150 µg/L | Lancet 2021 |
| Serum iron | 10–30 µmol/L | 15–25 µmol/L | BSH guidelines |
| TIBC | 45–80 µmol/L | 50–70 µmol/L | BSH guidelines |
| Transferrin saturation | 15–50% | 20–45% | BSH |
| Haemoglobin | 120–160 g/L (women) 130–170 g/L (men) | 130–150 g/L (women) 140–160 g/L (men) | WHO |
| MCV | 80–100 fL | 85–95 fL | BSH guidelines |
Note: "Optimal" ranges are based on published research into symptom resolution and functional outcomes, not population statistics. Individual targets may vary — discuss with your GP or a qualified practitioner.
7. What the GP Tests vs What You Can Get Privately
Your GP will typically only order iron studies if you present with clear symptoms and your full blood count shows abnormalities. Here's what you get at each level:
| MARKER | NHS (GP) | HELVY NUTRITION PANEL |
|---|---|---|
| Full blood count (FBC) | ✓ | ✓ |
| Ferritin | Sometimes | ✓ |
| Serum iron | Rarely | ✓ |
| TIBC | Rarely | ✓ |
| Transferrin saturation | Rarely | ✓ |
| Vitamin B12 | Sometimes | ✓ |
| Folate | Sometimes | ✓ |
| Vitamin D | Rarely | ✓ |
| Magnesium | ✗ | ✓ |
The Helvy Nutrition panel (£99) includes a full iron study alongside the other nutrients most commonly co-deficient with iron. The Essential panel (£129) includes ferritin as part of a broader health screen.
8. How to Interpret Your Results (With Examples)
Iron results are most useful when read as a pattern, not individual numbers. Here are four common scenarios:
PATTERN A: EARLY DEPLETION
Ferritin 18 µg/L · Serum iron 14 µmol/L · TIBC 72 µmol/L · TSAT 19% · Hb 128 g/L
Ferritin is low but haemoglobin is normal. GP says "you're fine." In reality, iron stores are depleted and symptoms (fatigue, brain fog) are likely already present. This is Stage 1.
PATTERN B: FUNCTIONAL DEFICIENCY
Ferritin 10 µg/L · Serum iron 8 µmol/L · TIBC 78 µmol/L · TSAT 10% · Hb 118 g/L
Ferritin depleted, TSAT below 16%, haemoglobin borderline. Stage 2 — iron-deficient erythropoiesis. Bone marrow is starved of iron. Supplementation should begin urgently and GP referral is warranted.
PATTERN C: IRON DEFICIENCY ANAEMIA
Ferritin 5 µg/L · Serum iron 5 µmol/L · TIBC 82 µmol/L · TSAT 6% · Hb 98 g/L · MCV 72 fL
Stage 3 — full iron deficiency anaemia. Low haemoglobin, low MCV (microcytic), depleted stores. GP investigation is essential to identify the cause (menstrual loss, GI bleeding, coeliac, etc.).
PATTERN D: FERRITIN ELEVATED BY INFLAMMATION
Ferritin 180 µg/L · Serum iron 9 µmol/L · TIBC 75 µmol/L · TSAT 12% · CRP 28 mg/L
Ferritin looks "normal" but is falsely elevated by inflammation (high CRP). Low serum iron and low TSAT reveal the true picture: functional iron deficiency masked by the acute-phase response. This is why a full iron panel matters.
9. The Ferritin Paradox: "Normal" Doesn't Mean Optimal
Most UK labs report ferritin as "normal" anywhere from 15 to 300 µg/L. This enormous range means a woman with ferritin of 16 gets the same "all clear" as someone with 200 — despite being functionally depleted.
Research published in The Lancet and the BMJ increasingly supports a functional threshold of 30–50 µg/L for symptom resolution, particularly for fatigue, cognitive function, and exercise capacity. The WHO (2020) updated its threshold to <15 µg/L for depletion and recommended <30 µg/L as a more sensitive cut-off when inflammation is present.
If your GP tells you a ferritin of 18 is "normal," they are technically correct by the lab range. But the published evidence suggests your body may need significantly more to function at its best. This is exactly the kind of insight a comprehensive blood panel reveals that a standard GP test often misses.
10. Common Causes of Iron Deficiency
Iron deficiency is always caused by one of three mechanisms: increased loss, reduced intake, or impaired absorption.
Increased loss
- Heavy menstrual periods (most common cause in premenopausal women)
- GI bleeding (ulcers, polyps, colorectal cancer, haemorrhoids)
- Regular blood donation
- Endurance exercise (foot-strike haemolysis, sweat losses)
Reduced intake
- Vegetarian or vegan diet without strategic iron-rich food choices
- Restrictive dieting or disordered eating
- Elderly populations with poor appetite
Impaired absorption
- Coeliac disease (NICE NG20) — damages the duodenum where most iron absorption occurs
- Inflammatory bowel disease (Crohn's, ulcerative colitis)
- Long-term PPI use (omeprazole, lansoprazole)
- H. pylori infection
- Gastric bypass surgery
Important: In men and postmenopausal women, iron deficiency always warrants investigation for GI blood loss. The NICE suspected cancer pathway (NG12) recommends urgent 2-week-wait referral for unexplained iron deficiency anaemia in this group.
11. What to Do if Your Iron Is Low
Treatment depends on the severity of deficiency and the underlying cause. These are general approaches — always discuss with your GP before starting supplementation.
Dietary changes
Best iron-rich foods: red meat, liver, mussels, sardines, dark leafy greens (spinach, kale), lentils, chickpeas, tofu, fortified cereals. Vitamin C dramatically increases non-haem iron absorption — pair plant-based iron sources with citrus, peppers, or tomatoes.
Avoid tea, coffee, and calcium-rich foods within 1 hour of iron-rich meals — they inhibit absorption by 40–60%.
Oral supplementation
Iron bisglycinate (chelated iron) is generally better tolerated than ferrous sulphate, with fewer GI side effects. Typical dose: 25–50 mg elemental iron daily, taken on an empty stomach with vitamin C. The BMJ reports that alternate-day dosing (rather than daily) may actually improve fractional absorption due to hepcidin cycling.
Never supplement iron without a blood test. Excess iron is toxic and cannot be excreted. Iron overload (haemochromatosis) affects approximately 1 in 200 people of Northern European descent.
IV iron infusion
For severe deficiency, malabsorption conditions, or when oral iron is not tolerated, your GP may refer you for intravenous iron (ferric carboxymaltose / Ferinject). A single infusion can replenish stores in 15–30 minutes. NICE recommends IV iron for iron deficiency anaemia in pregnancy when oral iron fails or is not tolerated.
12. When to Retest
Iron stores take time to replenish. The BSH recommends:
- 8–12 weeks after starting oral supplementation — this is the minimum time for ferritin to respond meaningfully
- 4–8 weeks after IV iron infusion — ferritin rises faster with IV
- Every 3–6 months during ongoing supplementation until ferritin stabilises above 50 µg/L
- Annually for high-risk groups (heavy periods, vegetarians, athletes) even when stores are optimal
Continue supplementation for at least 3 months after ferritin normalises to ensure stores are fully replenished, not just temporarily topped up.
13. When to See Your GP Urgently
Most iron deficiency is manageable with dietary changes and supplementation. However, book a GP appointment promptly if:
- Your haemoglobin is below 100 g/L (moderate-severe anaemia)
- You are a man or postmenopausal woman with unexplained iron deficiency
- You have blood in your stool, very dark stools, or unexplained weight loss
- You are pregnant and your ferritin is below 30 µg/L
- Your iron levels do not improve after 8–12 weeks of supplementation
- You experience chest pain, severe breathlessness, or palpitations
The NICE suspected cancer pathway (NG12) flags unexplained iron deficiency anaemia in men of any age and postmenopausal women as a potential indicator of colorectal cancer. This isn't cause for alarm — many causes are benign — but prompt investigation is important.
14. Frequently Asked Questions
Can I have iron deficiency without anaemia?
Yes — and it's extremely common. Iron depletion (Stage 1) and iron-deficient erythropoiesis (Stage 2) both cause symptoms like fatigue, brain fog, and hair loss while haemoglobin remains within the normal range. The WHO estimates that for every case of iron deficiency anaemia, there are 2–3 people with non-anaemic iron deficiency.
Do I need to fast before an iron blood test?
Fasting is recommended for the most accurate serum iron and transferrin saturation results (ideally morning, before eating). Ferritin is not significantly affected by recent meals, but a fasting morning sample gives the most consistent results across all markers.
How long does it take to correct iron deficiency?
Haemoglobin typically improves within 2–4 weeks of starting supplementation. Ferritin takes 8–12 weeks to show meaningful improvement and 3–6 months to fully normalise. The BSH recommends continuing supplementation for 3 months after ferritin normalises to ensure stores are properly replenished.
Is it safe to take iron supplements without a blood test?
No. Iron is one of the few minerals where excess is genuinely dangerous. Hereditary haemochromatosis (iron overload) affects approximately 1 in 200 people of Northern European descent, and unnecessary iron supplementation can cause organ damage. Always confirm deficiency with a blood test first.
What's the best form of iron supplement?
Iron bisglycinate (chelated iron) is generally the best-tolerated form with the fewest GI side effects. Ferrous sulphate (the NHS standard) is cheaper but causes constipation and nausea in up to 40% of people. Both are effective at raising ferritin — compliance matters more than the form you choose.
Can too much iron be harmful?
Yes. Excess iron generates oxidative stress and can damage the liver, heart, and pancreas. Ferritin above 300 µg/L in men or 200 µg/L in women warrants investigation for iron overload. This is why blood testing before and during supplementation is essential — iron is not a "more is better" nutrient.
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