Iron deficiency anaemia: understanding ferritin and your blood test results

Iron deficiency is one of the most common nutritional deficiencies in the world, and one of the most common reasons people order a private blood test in the UK. Symptoms like fatigue, breathlessness, pale skin, headaches, brittle nails, hair shedding, cold hands and feet, and restless legs can all be linked to low iron — but they overlap with many other conditions, which is exactly why blood tests matter.

If you’re here because you’ve been told you have low iron or low ferritin, or because you’re wondering whether your symptoms could be iron-related, this guide explains the main markers in plain English. Please also read it as background and speak to your GP. The cause of iron deficiency matters more than the number itself, and getting that right is GP territory rather than something to manage alone.

Iron deficiency and iron deficiency anaemia aren’t the same thing

This is the most important point to get clear, and it’s a common source of confusion.

Iron deficiency means your body’s iron stores are low. The most sensitive blood marker is ferritin, a protein that stores iron. You can have low ferritin — and feel the symptoms of it — without yet being anaemic.

Iron deficiency anaemia (IDA) is what happens when iron deficiency has progressed far enough that the body can’t make enough healthy red blood cells. Haemoglobin then falls below the reference range, and the blood count picks it up.

This matters because:

• You can be iron-deficient without being anaemic. Low ferritin and the symptoms of low iron can appear well before haemoglobin drops. Treating it earlier is often easier and quicker than waiting for full-blown anaemia.
• You can have iron deficiency anaemia with more than one cause. It’s not always purely dietary. A combination — for example a low-iron diet plus heavy periods, or low intake plus a gut absorption issue — is common.
• The investigation pathway is the same in principle: find out why iron is low, then treat the cause as well as the deficiency.

The NHS notes that iron deficiency anaemia is one of the most common forms of anaemia in the UK, and is most often caused by blood loss or low dietary intake.

What an iron blood test typically measures

An iron panel can vary between labs, but it commonly includes:

• Ferritin — a protein that stores iron. Low ferritin is the most sensitive early marker of iron deficiency, and the most important single number on an iron panel for most adults.
• Serum iron — the amount of iron circulating in the blood at the time of the test. Varies through the day and with recent food.
• Transferrin — a protein that transports iron in the blood. Often reported together with, or as, total iron binding capacity (TIBC), a related measurement.
• Transferrin saturation — the percentage of transferrin currently carrying iron. Calculated from serum iron and TIBC.

These markers are interpreted together, and they’re often interpreted alongside a full blood count, which shows whether iron deficiency has progressed to anaemia.

Ferritin explained

Ferritin is the central marker on most iron panels. It reflects your body’s iron stores: how much iron you have in reserve, not just how much is circulating in your blood at the moment of the test.

The lower limit for ferritin varies between UK labs, and some clinicians use higher thresholds when symptoms are present. The lab report will show the reference range used, but interpretation still needs clinical context.

There’s one really important caveat about ferritin that often gets missed: ferritin is also an acute-phase protein. That means it rises with inflammation, infection, liver disease, and recent illness. So someone who genuinely has low iron stores but also has ongoing inflammation may show a “normal-looking” ferritin that’s actually misleadingly normal.

This is why ferritin is usually interpreted alongside CRP (a marker of inflammation) and the wider clinical picture. A ferritin that looks borderline-normal with raised CRP is a different situation to the same number with no inflammation present. Your GP will factor this in.

Serum iron, transferrin and transferrin saturation

The other iron markers add useful context to ferritin, though none of them is as sensitive on its own.

Serum iron alone is rarely useful. It varies significantly through the day, and recent meals — particularly iron-rich foods or supplements — can change it. A single serum iron result, in isolation, doesn’t tell you much about your iron stores.

Transferrin (or TIBC) typically rises when iron stores are low. The body makes more transport protein to grab whatever iron is available. A raised transferrin or TIBC alongside a low ferritin is a coherent picture.

Transferrin saturation gives a useful ratio — how much of your iron-transport capacity is actually being used. Low saturation alongside low ferritin is the classic pattern of iron deficiency:

• Low ferritin + low transferrin saturation + raised TIBC is the classic iron-deficiency picture.
• Normal ferritin in someone with symptoms and high inflammation markers is harder to interpret — and is exactly where GP judgement matters.

What low iron may suggest

Iron deficiency has several common causes, and the cause matters more than the number on the report:

• Inadequate dietary intake — vegetarian, vegan or low-meat diets can be low in well-absorbed iron unless deliberately planned. (Plant sources of iron are real but less easily absorbed than iron from meat.)
• Blood loss — heavy periods are the most common cause in pre-menopausal women in the UK. Gastrointestinal blood loss, including from ulcers or bowel conditions, is an important consideration in men and post-menopausal women. Frequent blood donation and recent surgery can also contribute.
• Increased demand — pregnancy is a common cause; rapid growth in adolescence and the demands of endurance training can also lower iron.
• Poor absorption — coeliac disease, inflammatory bowel disease, gastritis, recent gastric surgery, and some medications (including certain heartburn medications) can reduce how much iron the gut absorbs from food.
• Combination of factors — for example a low-iron diet alongside heavy periods, or moderate intake alongside a gut absorption issue.

This is exactly why iron deficiency isn’t a problem to manage alone. Heavy menstrual bleeding and unexplained gastrointestinal blood loss are common causes that need investigation, not just supplements. Your GP can take a proper history, examine you, and decide whether further tests — such as coeliac screening, stool tests, or referral for bowel investigation — are appropriate.

For men and post-menopausal women in particular, iron deficiency is less common than in pre-menopausal women, and the cause more often needs careful investigation. Your GP may arrange further tests to find the cause of unexplained iron deficiency in these groups, sometimes including referral.

What raised iron or raised ferritin may suggest

Raised iron markers are less commonly investigated than low ones, but still worth understanding.

A raised ferritin without raised iron or saturation is most often a reflection of inflammation, infection, or liver disease — ferritin rising as an acute-phase protein rather than reflecting true iron overload.

Raised ferritin with raised transferrin saturation is a different pattern and needs GP investigation. Causes can include:

• Recent iron supplementation or transfusion
• Liver disease
• Haemochromatosis, a genetic condition that causes the body to absorb too much iron from food. It is one of the more common genetic conditions in the UK and is often underdiagnosed.

This is one reason iron supplementation should never be started without a proper iron panel and GP advice. For people with haemochromatosis, iron supplements can make the underlying problem significantly worse.

What iron blood tests can’t tell you

Iron tests are useful, but they have limits:

• One result doesn’t tell you the cause of iron deficiency. The number is a starting point; the cause is what determines treatment.
• Ferritin can be misleadingly normal in the presence of inflammation or liver disease. This is why ferritin is interpreted alongside CRP and the wider picture rather than in isolation.
• Iron levels fluctuate. Repeat testing is sometimes needed, particularly when results are borderline or when treatment is being monitored.
• The tests don’t replace clinical examination and history-taking. Symptoms, periods, diet, gut health and family history all shape interpretation.
• They don’t replace your GP’s judgement. Only a clinician with your full history can put the markers in context.

If your iron markers are outside the reference range, the next step is a GP appointment — not extensive online research about specific conditions or supplement protocols. Your GP can examine you, take a proper history, and arrange follow-up tests where they’re needed.

Symptoms and overlap with other conditions

Iron deficiency is one of the most common causes of fatigue in the UK, but it’s far from the only one. Many of the symptoms that prompt an iron test also overlap with:

• Thyroid problems
• Sleep disorders or chronic sleep deprivation
• Depression and anxiety
• Vitamin D and B12 deficiency
• Chronic infections or autoimmune conditions
• Kidney conditions — which can cause anaemia for reasons other than iron

This is why iron status is most useful when interpreted alongside other markers rather than as a standalone test. A symptom of fatigue without a single clear cause often benefits from a broader review, not just one number.

A note on iron supplementation

This is the area where readers most want a clear answer, and it’s the area where it’s most important to route to a clinician.

Iron supplements come in many forms — ferrous sulphate, ferrous fumarate, ferrous gluconate, liquid preparations, modified-release versions — with different doses, side effects and tolerability profiles. The right form and dose depends on the severity of deficiency, the cause, your gut tolerance, what other medications you take, and your individual circumstances.

Over-supplementation is harmful. This is particularly important for people with haemochromatosis, who can be made significantly worse by self-prescribed iron.

If you’re considering iron supplementation, speak to your GP or a pharmacist first. The right approach depends on your blood test results, the cause of any deficiency, and your individual situation — not on what works for someone else.

Should you book a single iron panel or a broader health check?

This depends on what you’re trying to understand.

A single iron panel may make sense if:

• Your GP has previously asked you to monitor iron levels.
• You have a known reason to track ferritin — for example, previous iron deficiency, regular blood donation, or a vegetarian or vegan diet you’re managing actively.
• You want a focused recheck after treatment.

You can compare options on the private blood tests page.

A broader health check is often more useful if:

• You have symptoms but no clear cause — iron is most meaningfully interpreted alongside full blood count, thyroid function, CRP and other markers.
• You’d like a fuller baseline rather than just one panel.
• You want a structured review with time to discuss multiple aspects of your health.

The Advanced Health Check is a natural starting point. The 5 essential blood tests for adults guide and why health screening matters post give wider context if you’re new to private blood testing.

If you’re not sure which option suits you, contact us and we can help you choose.

When to speak to your GP

Speak to your GP if you have symptoms of iron deficiency, or if any of your iron or haemoglobin markers are outside the reference range. The cause of low iron is the real question — and that’s GP territory.

Speak to your GP promptly if you have:

• Heavy periods that are affecting your daily life — this is the most common cause of iron deficiency anaemia in pre-menopausal women in the UK, and there are several ways your GP can help.
• Blood in your stool, dark or tarry stools, or blood when vomiting — these are signs of gastrointestinal blood loss that need investigation.
• Unexplained iron deficiency, particularly if you’re a man or a post-menopausal woman — the cause often needs more thorough investigation and sometimes specialist referral.
• Significant fatigue, breathlessness, dizziness, fainting, or chest pain — these can be signs of more advanced anaemia and need prompt review.

If you’re pregnant or planning a pregnancy, your midwife or GP is the right person to manage iron — requirements increase in pregnancy and routine antenatal care includes iron monitoring.

A private health check is designed for adults who currently feel well or want a baseline. It can usefully complement NHS care and help you prepare for a more focused conversation with your GP. It doesn’t replace medical care if you’re unwell.

In a life-threatening emergency — severe bleeding, fainting, severe weakness, chest pain — call 999 immediately. For urgent advice that isn’t an emergency, NHS 111 is available online or by phone.

The number is a starting point

Iron blood tests are useful because ferritin, transferrin and iron together can flag patterns that may be worth acting on. Low ferritin can prompt earlier action; classic iron-deficiency patterns can guide investigation; and a clear iron picture can save a lot of guesswork around symptoms like fatigue.

But the markers are a starting point, not an answer. The cause of any iron problem matters more than the number itself, and getting the cause right — and any treatment right — is GP territory. The most useful interpretation comes from looking at iron markers alongside your symptoms, history, periods, gut health, and other tests.

If you want to check iron status as part of a broader review, the Advanced Health Check is a strong option. If you’re unsure which option is right for you, contact us and we can help you find the most appropriate starting point. Appointments are available from our Kingston upon Thames and Crawley clinics.