What Is Ferritin? Your Iron Stores, Explained (And Why Your GP Checks It)

Ferritin is the protein your body uses to store iron inside its cells - it reflects your total iron reserves rather than the iron circulating in your blood. Because ferritin falls before haemoglobin does, it is often the earliest sign that iron stores are running low, sometimes weeks before a standard anaemia test would flag anything. This guide explains what ferritin is, what your result means, and how it differs from serum iron and haemoglobin.

What is ferritin, and what does a ferritin test measure?

Ferritin is a protein that stores iron inside your cells - not iron itself. Each ferritin molecule consists of a hollow protein shell (apoferritin) capable of holding up to roughly 4,500 iron atoms, which gives the body a substantial buffer to draw on when iron demand rises. Most ferritin is held in the liver, spleen, and bone marrow.

A ferritin blood test measures the small amount of ferritin that circulates in your bloodstream, which acts as an indirect marker of your total iron reserves. According to NHS guidance on iron, this is one of the reasons a low result can appear even when other iron markers still look normal.

Short answer

Ferritin reflects how much iron your body has in storage. In UK practice, ferritin below around 15 µg/L typically indicates depleted iron stores, the threshold defined in SACN's 2010 Iron and Health report. However, symptoms such as fatigue, hair shedding, and brain fog can appear at higher levels - often below 30 µg/L - well before anaemia develops.

Evidence at a glance

• Established: SACN's 2010 Iron and Health report defines serum ferritin below 15 µg/L as the threshold for depleted iron stores; typical laboratory normal ranges run 15-300 µg/L for women.
• Established: NHS dietary guidance sets the iron requirement at 14.8 mg/day for women aged 19-50 and 8.7 mg/day for men and post-menopausal women.
• Emerging: Evidence links ferritin below 30-40 µg/L with hair shedding (telogen effluxion) even without anaemia (Trost et al., Journal of the American Academy of Dermatology), though it is one of several contributing factors.
• Limited: The idea that whole-food iron sources such as spirulina are absorbed better than supplements because of their cellular structure is not yet supported by controlled human trials.

Why does the body store iron as ferritin?

The body stores iron as ferritin because iron is essential but also potentially harmful when left unmanaged. The body cannot actively excrete excess iron, so balance depends entirely on regulating absorption, storage, and release. Too little iron impairs oxygen transport and energy production; too much can drive oxidative stress and cellular damage.

Ferritin acts as a buffering system. It locks iron away safely when supply exceeds immediate demand and releases it when needed, keeping a steady supply available for red blood cell production, oxygen transport, and cellular energy processes. Without this storage layer, iron availability would swing sharply with every meal and every short-term demand.

A person can have normal circulating iron in the short term while ferritin is already low, meaning reserves are quietly running down. For the wider context of how stores connect to symptoms, our guide to low ferritin and iron absorption covers the day-to-day picture in more detail.

Ferritin vs iron vs haemoglobin: what is the difference?

Ferritin stores iron, serum iron transports it, and haemoglobin uses it to carry oxygen - three different jobs within one connected system. Many people assume iron status is a single number, but each marker measures a separate stage.

• Ferritin → storage. Reflects iron reserves held inside cells; the most sensitive early marker.
• Serum iron → transport. Iron circulating in the blood at the moment of the test - highly variable through the day and less reliable for assessing stores.
• Haemoglobin → functional use. Iron incorporated into red blood cells to carry oxygen. NHS thresholds define anaemia as haemoglobin below 120 g/L in women and 130 g/L in men.

This sequencing is why ferritin matters. Stored iron is drawn down first, so ferritin falls early. Only once reserves are exhausted does red blood cell production - and therefore haemoglobin - begin to suffer. A normal haemoglobin result does not rule out low iron stores.

How is iron managed in the body?

Iron metabolism works as a coordinated, tightly regulated system. Following one iron atom through the body makes the role of ferritin clearer:

• Iron is absorbed through the wall of the small intestine - non-haem iron via the DMT1 transporter once reduced to its ferrous (Fe²⁺) form.
• It enters the bloodstream and binds to transferrin, the protein that carries it to tissues.
• It is delivered to bone marrow for red blood cell production and to other tissues for immediate use.
• Excess iron is stored inside cells as ferritin in the liver, spleen, and bone marrow.
• The body recycles roughly 20-25 mg of iron daily from old red blood cells - far more than the 1-2 mg absorbed from food.

Hepcidin: the hormone that controls the gates

Hepcidin is a peptide hormone produced by the liver that regulates how much iron enters circulation. It works by binding to ferroportin - the export channel that releases iron from intestinal cells and from the macrophages that recycle old red blood cells - and causing it to be broken down.

When iron stores are high, or during infection and inflammation, hepcidin rises and blocks ferroportin, so iron stays trapped inside cells and less enters the blood. When the body is iron-deficient, hepcidin falls, ferroportin stays open, and more iron is absorbed and released. This is one reason iron deficiency is harder to correct during illness: inflammation keeps hepcidin elevated even when ferritin is low.

What does low ferritin mean?

Low ferritin means your iron stores are reduced - but it does not automatically mean you have anaemia. In many cases it reflects an earlier stage of depletion, which is precisely why ferritin is so useful as a warning marker.

Iron deficiency typically progresses through three stages described in NICE CKS guidance on iron deficiency and SACN's report:

• Stage 1 - iron depletion. Ferritin falls below 15 µg/L but haemoglobin stays normal. Symptoms such as fatigue, brain fog, and hair shedding can already appear.
• Stage 2 - iron-deficient erythropoiesis. Stores are exhausted, transferrin saturation drops below about 20%, and red blood cell production becomes impaired - though haemoglobin still sits above the anaemia threshold.
• Stage 3 - iron deficiency anaemia. Haemoglobin drops below the NHS threshold, and red blood cells become small (microcytic) and pale (hypochromic). This is the point a GP diagnoses anaemia.

NICE guidance recommends investigating low ferritin even without anaemia, particularly in women with fatigue. A serum ferritin test is the first-line investigation, and unexplained iron deficiency in men or post-menopausal women should always be assessed for a source of blood loss.

What influences ferritin levels?

Ferritin levels are shaped by intake, losses, absorption, and demand acting together - not simply by how much iron you eat.

Intake

Dietary iron intake matters most in low-iron and poorly planned plant-based diets. The UK National Diet and Nutrition Survey found that around 27% of girls aged 11-18 and roughly 12% of adult women have low iron intakes. Even so, intake alone rarely explains the whole picture.

Losses

Iron loss is a leading cause of low ferritin, especially in women. A typical menstrual cycle loses around 15-25 mg of iron, and heavy periods can push that to 80-200 ml of blood loss per cycle. Blood donation removes roughly 200-250 mg of iron per session, and gastrointestinal blood loss is a common cause that needs investigation.

Absorption

You can eat enough iron yet absorb very little of it. Non-haem iron - the form in plants, eggs, and most supplements - is absorbed at only 2-20% depending on the meal, while haem iron from red meat is absorbed at 15-35% largely regardless of context. Vitamin C reduces ferric iron (Fe³⁺) to the absorbable ferrous (Fe²⁺) form and can increase non-haem absorption two- to three-fold (Hallberg et al., American Journal of Clinical Nutrition). Tannins in tea reduce non-haem absorption by 60-90% in the same meal, which is why the BDA suggests a one-hour gap between an iron-rich meal and tea or coffee.

Demand

Pregnancy, rapid growth, and high physical activity all raise iron demand. Endurance athletes are a notable group: ferritin below 30-40 µg/L is associated with impaired aerobic performance even before anaemia develops, because of foot-strike haemolysis and increased losses during exercise.

Why iron is more than "just iron"

Iron metabolism depends on several nutrients and processes working together, not on iron alone. Increasing iron intake will not always resolve low ferritin if another part of the system is the bottleneck.

• Vitamin C - enhances non-haem iron absorption.
• Copper - involved in iron transport.
• B12 and folate - support normal red blood cell formation.
• Gut environment - influences how effectively iron is absorbed.

For iron to actually raise ferritin, it must be absorbed effectively, transported by transferrin, stored appropriately, and used correctly. Disrupt one step - through low vitamin C, ongoing blood loss, or elevated hepcidin from inflammation - and the whole system underperforms.

Where does spirulina fit into iron nutrition?

Spirulina is a nutrient-dense microalgae that provides non-haem iron alongside protein, B-vitamins, and trace elements within a single cellular matrix. USDA food composition data records roughly 2 mg of iron per 3 g serving of dried spirulina - about 14% of the daily reference value for women aged 19-50.

Because spirulina lacks a tough cellulose cell wall, its protein is highly bioavailable, with human studies estimating 83-90% digestibility. The iron itself is held within the cellular structure of the microalgae; whether this affects iron bioavailability in humans compared with isolated iron salts requires further study, so it is best understood as a whole-food iron contribution rather than a proven advantage.

Iron nutrition depends on consistent nutrient composition - controlled cultivation and batch testing help provide a more reproducible Spirulina profile than material sourced from variable outdoor environments. Algoglobin Alphyca builds on this approach by combining Spirulina-derived iron with vitamin C, B12, folate and copper—nutrients that contribute to normal iron metabolism and red blood cell formation.

Practical ways to support healthy ferritin levels

Supporting ferritin means improving the whole iron system, not only raising iron intake. The following adjustments are grounded in NHS and BDA guidance:

• Pair iron with vitamin C. 50-100 mg of vitamin C alongside an iron-containing meal is enough to meaningfully improve non-haem absorption.
• Space tea and coffee from meals. Wait at least an hour after an iron-rich meal to avoid the tannin effect.
• Support gut health. A healthy gut lining is where absorption happens, so digestive comfort is part of the picture.
• Include whole-food sources. They deliver iron alongside supporting micronutrients rather than in isolation.
• Monitor over time. Ferritin changes gradually; retesting at GP-advised intervals shows the trend, not a single snapshot.

Iron supplements should be considered with a GP's input. Ferrous sulphate is the standard NHS-prescribed form and the most absorbed, while ferrous gluconate and fumarate are slightly gentler on digestion.

When should you speak to a GP about ferritin?

Speak to your GP if a blood test shows low ferritin, if you have persistent fatigue or unexplained symptoms, if you are pregnant or planning pregnancy, or if there is any concern about absorption problems or blood loss. NICE recommends GP assessment for suspected iron deficiency, with ferritin, a full blood count, and transferrin saturation used to confirm a diagnosis.

Ferritin is a useful marker, but it is an acute-phase protein that rises during infection and inflammation, which can mask true depletion. That is why it must be read in the context of your overall health rather than in isolation, and why self-treatment without guidance is not advisable.

FAQ

What is ferritin in simple terms?

Ferritin is a protein that stores iron inside your cells and reflects your total iron reserves. Its hollow protein shell can hold up to around 4,500 iron atoms, releasing them when the body needs iron for oxygen transport or red blood cell production. A ferritin test is the most sensitive early indicator of declining iron status.

Is ferritin the same as iron?

No - ferritin is the protein that stores iron, while iron is the mineral itself. Iron circulates in the blood and is used to build haemoglobin, whereas ferritin represents what your body has set aside in storage. You can have normal circulating iron while ferritin is already low.

Can ferritin be low without anaemia?

Yes - ferritin typically falls before haemoglobin is affected, so iron stores can be depleted while a standard anaemia test still reads normal. This is Stage 1 iron depletion, and symptoms such as fatigue and hair shedding can appear at this point. NICE guidance recommends investigating low ferritin in women with fatigue even when haemoglobin is normal.

What ferritin level is considered low in the UK?

SACN's 2010 Iron and Health report defines ferritin below 15 µg/L as depleted iron stores, though symptoms often appear below 30 µg/L. Typical laboratory normal ranges are 15-300 µg/L for women and 15-400 µg/L for men. Because reference ranges vary between laboratories, your result should be interpreted by your GP.

What causes low ferritin?

Low ferritin usually reflects reduced iron stores caused by low intake, blood loss, poor absorption, or increased demand. Common drivers include menstruation, pregnancy, blood donation, and gastrointestinal blood loss. Inflammation can also keep hepcidin elevated, suppressing absorption even when stores are low.

Key takeaways

• Each ferritin molecule consists of a protein shell (apoferritin) that can hold up to roughly 4,500 iron atoms, giving the body a substantial iron buffer.
• Ferritin below 15 µg/L indicates depleted iron stores (SACN 2010), but symptoms can appear below 30 µg/L while haemoglobin remains normal.
• Hepcidin, a liver hormone, blocks ferroportin to limit iron absorption when stores are high or during inflammation - which is why deficiency is harder to treat during illness.
• Non-haem iron is absorbed at only 2-20% versus 15-35% for haem iron; vitamin C can increase non-haem absorption two- to three-fold.
• Tannins in tea cut non-haem iron absorption by 60-90% in the same meal, so the BDA advises a one-hour gap from iron-rich meals.
• NICE recommends investigating low ferritin even without anaemia, especially in women with fatigue, using ferritin as the first-line test.
• Spirulina provides around 2 mg of non-haem iron per 3 g serving (USDA data); whether its cellular matrix improves bioavailability in humans is not yet established.

 

Medical note: This article is for general education and is not a substitute for medical advice. Ferritin results should always be interpreted by a GP alongside your full clinical picture. If a blood test shows low ferritin, or you have unexplained fatigue, speak to your GP before making changes.