Ferritin or Iron? How to Check Your Body’s Iron Stores

“My iron level is normal, so I can’t have anaemia” is one of the most common incorrect conclusions drawn from blood test results. Serum iron concentrations fluctuate throughout the day and depend on your most recent meal, supplements, inflammation and the time of day. They reveal very little about how much iron your body has in reserve. That is what ferritin is used to assess. Understanding the difference between these two tests, and how TIBC, transferrin and CRP complement them, helps prevent both missed iron deficiency and unnecessary supplementation.

How your body manages iron

An adult has 3–4 g of iron in their body. Approximately two-thirds is found in the haemoglobin of red blood cells, with the remainder present in muscle myoglobin, enzymes and iron stores: ferritin and haemosiderin in the liver, spleen and bone marrow. Only 1–2 mg of iron is absorbed from food each day, and approximately the same amount is lost through the shedding of epithelial cells. Menstruation, pregnancy, gastrointestinal bleeding and intensive exercise increase iron losses. When the balance becomes negative, the body first uses up its reserves (ferritin falls). Next, iron becomes insufficient for haemoglobin production (transferrin saturation falls, RDW rises, and MCV and MCH decrease). Only in the final stage does haemoglobin fall and anaemia develop. Months may pass between the first and last stages, during which a person may already experience symptoms while their full blood count remains normal.

Ferritin: a measure of iron stores

Ferritin is an iron-storage protein, and its concentration in the blood reflects the body’s iron reserves. Low ferritin is a strong indicator of iron deficiency, as there are few other common causes. The lower limit of the reference range in most laboratories is approximately 10–30 ng/ml. However, many specialists consider values below 30 ng/ml to indicate depleted iron stores, while symptoms such as fatigue, hair loss and restless legs syndrome may occur at levels below 50 ng/ml. Target values may be higher for pregnant women and endurance athletes.

There is an important limitation: ferritin is an acute-phase protein. Its concentration increases during infections, inflammatory conditions, liver disease, obesity, following strenuous exercise and in cancer. Someone with iron deficiency and concurrent inflammation may therefore have a ferritin result within the reference range. This is why ferritin is often measured alongside C-reactive protein (CRP). When CRP is elevated, a ferritin concentration of up to 100 ng/ml does not exclude iron deficiency.

Serum iron: a snapshot, not an assessment of reserves

Serum iron concentrations follow a daily rhythm (generally highest in the morning), rise after a meat-based meal or an iron tablet, sometimes for many hours, and fall during infections. The same person may have a normal result in the morning and a value below the reference range in the afternoon. Serum iron alone, without ferritin and TIBC, should not be used as the basis for clinical decisions. However, it is useful for calculating transferrin saturation and investigating iron overload.

TIBC, transferrin and transferrin saturation

Transferrin is the protein responsible for transporting iron in the blood. TIBC (total iron-binding capacity) indicates how much iron transferrin could potentially bind. In iron deficiency, the body produces more transferrin (TIBC increases), but relatively little iron is bound to it: transferrin saturation (TSAT = serum iron / TIBC × 100%) falls below 15–20%. In anaemia of chronic disease, TIBC is low or normal and ferritin is elevated, helping distinguish these two conditions. In haemochromatosis (iron overload), transferrin saturation exceeds 45% and is a more sensitive screening indicator than ferritin alone.

Soluble transferrin receptor (sTfR)

This additional test is useful when inflammation makes ferritin difficult to interpret. sTfR increases in iron deficiency and is not significantly affected by inflammation. It is not routinely performed but can help resolve diagnostically challenging cases.

How to interpret a combination of results

Symptoms of iron deficiency without anaemia

Fatigue disproportionate to physical exertion, reduced exercise tolerance, hair loss, brittle nails, cracks at the corners of the mouth, restless legs syndrome, difficulty concentrating, cravings for ice or unusual non-food substances (pica), and more frequent infections. These symptoms often develop many months before haemoglobin levels fall and may improve once iron stores are replenished.

Who should have their ferritin checked?

Women who menstruate (particularly those with heavy periods), women who are pregnant or planning pregnancy, people following plant-based diets, endurance athletes, blood donors, people who have undergone bariatric surgery, those with coeliac disease or inflammatory bowel disease, people taking antacids or proton pump inhibitors (which reduce iron absorption), and anyone experiencing unexplained fatigue or hair loss. In men and postmenopausal women, iron deficiency should always prompt investigation for possible gastrointestinal blood loss.

Iron supplementation: when to start, dosage and monitoring

The decision to start iron supplementation should be made by a doctor based on ferritin levels rather than serum iron alone. Oral iron is absorbed more efficiently when taken on alternate days, in the morning, on an empty stomach or with vitamin C, and separately from coffee, tea, dairy products and antacids. Replenishing iron stores takes 3–6 months. Rechecking ferritin and a full blood count after 8–12 weeks helps determine whether treatment is effective. Taking iron supplements ‘just in case’, without testing, can be harmful: excess iron damages the liver, pancreas and heart, and is particularly dangerous for people with undiagnosed haemochromatosis.

Preparing for the tests

Have your blood taken in the morning after fasting, with a 24–48-hour break from iron supplements (otherwise serum iron and TSAT may be falsely elevated). Avoid testing during an infection or immediately after strenuous exercise (which can increase ferritin). Minimum test panel: full blood count, Ferritin – laboratory test and C-reactive protein (CRP). Comprehensive panel: additionally, Iron – laboratory test, TIBC, transferrin saturation, Vitamin B12 – laboratory test and folate.

Frequently asked questions

My ferritin is 20 ng/ml, which the laboratory considers ‘normal’, but I have symptoms. What should I do?

Laboratory reference ranges describe a population, not necessarily optimal values. A ferritin level of 20 ng/ml indicates depleted iron stores. If you have this result alongside symptoms, you should consult a doctor, who will decide whether supplementation is appropriate.

Does high serum iron mean I have too much iron?

No. It often reflects the time of day, a recent meal or an iron tablet. Iron overload is suggested by elevated ferritin together with transferrin saturation above 45%.

Can I have a ferritin test without a referral?

Yes. Ferritin – laboratory test, Iron – laboratory test, TIBC, transferrin and full blood count are available privately, including as part of ready-made test packages.

Sources

• Pasricha SR et al., The Lancet, Iron deficiency, 2021. doi.org
• WHO, WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations, 2020. who.int
• Snook J. et al., Gut, British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults, 2021. gut.bmj.com