Ferritin: When Too Low and Too High Both Matter

Ferritin is one of the few laboratory values where both ends of the range mean something, and where the middle can still be wrong. A result of 18 ng/mL in a tired woman with thinning hair explains most of what she came in about. A result of 400 in a man with a fatty liver explains nothing until you know why it is 400. And a 60 in someone inflamed may be concealing a real deficiency, because the protein that reports iron stores is also an acute-phase reactant. Reading it means knowing which of its two jobs it was doing the day the blood was drawn.

What ferritin actually is

Ferritin is a hollow protein shell that stores iron in a soluble, non-reactive form. Free iron is dangerous — it catalyses the production of hydroxyl radicals — so cells keep it locked inside ferritin and release it only when needed. Most body ferritin sits inside cells, particularly in liver, spleen and bone marrow. What a blood test measures is the small fraction that leaks into serum, which under stable conditions is proportional to total stores.

That proportionality is the whole basis of the test, and it holds only when nothing else is driving ferritin production. Inflammatory cytokines, chiefly interleukin-6, increase ferritin synthesis directly as part of the innate immune response — hiding iron from pathogens that need it to grow. So serum ferritin rises with inflammation independently of iron status (Suchdev et al., Am J Clin Nutr 2017). One protein, two jobs, one number.

Low ferritin, before anaemia

Iron depletion happens in stages. Stores empty first — ferritin falls. Then transport iron falls and the marrow starts running short, which shows up as reduced transferrin saturation and rising transferrin. Only at the end does haemoglobin drop far enough to be called anaemia.

Symptoms do not wait for the last stage, which is why "your haemoglobin is normal" is not the reassurance it is offered as. Below roughly 30 ng/mL, stores are depleted, and the common presentation is fatigue disproportionate to effort, hair shedding, exercise intolerance with breathlessness at loads that used to be easy, restless legs, cold hands and feet, brain fog, and occasionally pica — a compulsion to chew ice being the classic version. A randomised trial of iron in non-anaemic women with unexplained fatigue found the treated group improved, which is the direct evidence that low stores alone can produce symptoms (Verdon et al., BMJ 2003).

Why those particular symptoms

The pattern makes sense once you stop thinking of iron as only the thing inside haemoglobin. Iron is a cofactor in the electron transport chain, so mitochondrial ATP production falls before red cell count does — hence fatigue while the blood count still looks fine. Hair follicles are among the fastest-dividing tissues in the body and are early casualties when a rate-limiting cofactor runs short. And iron is required for tyrosine hydroxylase, the rate-limiting enzyme of dopamine synthesis, which is the accepted explanation for restless legs improving with repletion.

The symptom list overlaps almost completely with hypothyroidism, so the two are worth checking together rather than sequentially — see the thyroid axis. Iron is also required by thyroid peroxidase, so one deficiency can constrain both systems.

Seeing through the inflammation confound

This is where most ferritin interpretation goes wrong. A person with active inflammation — obesity, autoimmune disease, recent infection, hard training in the days before the draw — can have depleted iron stores and a ferritin in the normal band, because the inflammatory drive and the depletion are pushing in opposite directions.

Three companion tests resolve it. Transferrin saturation is the most useful: it reports the proportion of transport protein currently carrying iron, and it falls in deficiency while inflammation does not raise it — the reasoning is set out in iron saturation. hs-CRP tells you whether the inflammatory confound is present at all; if it is raised, ferritin should be read upward-adjusted rather than at face value (hs-CRP explained). Transferrin itself rises in true deficiency, giving a third independent signal.

The practical rule: a borderline ferritin with a low transferrin saturation and a raised CRP should be treated as probable deficiency, not as normal.

High ferritin, in order of likelihood

Elevated ferritin triggers an immediate association with haemochromatosis, and haemochromatosis is almost never the answer. Ranked by how often each actually explains a raised result in an adult:

• Metabolic dysfunction and fatty liver — by a wide margin the commonest cause. Hepatic steatosis and insulin resistance raise ferritin with normal or near-normal transferrin saturation. GGT often moves in parallel and is a useful companion.
• Inflammation, acute or chronic, including recent infection.
• Alcohol, which raises ferritin both directly and through liver effects.
• Hereditary haemochromatosis — genuine iron overload, affecting roughly 1 in 200 people of northern European ancestry.
• Excess supplementation or repeated transfusions.
• Malignancy, rare, and rarely the sole abnormality.

The test that separates overload from everything else is transferrin saturation. Genuine iron overload raises it markedly; inflammatory and metabolic causes generally do not. Very high ferritin, above roughly 500 ng/mL, or any raised ferritin with a high saturation, warrants proper evaluation including genetic testing (Powell et al., Lancet 2016). Untreated haemochromatosis deposits iron in liver, pancreas, heart and joints, and it is entirely manageable when found early — which is the argument for taking a persistently high ferritin seriously rather than repeating it hopefully.

Find the cause before you treat the number

Iron deficiency in an adult is a symptom, not a diagnosis. Iron is not excreted in any regulated way; if stores are low, either intake was inadequate or blood was lost.

In menstruating women, heavy or lengthening periods are the usual explanation and the one most often normalised by the person experiencing it — see why periods get heavier in your 40s. In men and postmenopausal women, unexplained iron deficiency is a different matter: the default assumption is gastrointestinal blood loss until proven otherwise, which means investigation, not a supplement. Endurance athletes lose iron by several small routes at once, vegetarians absorb non-haem iron less efficiently, and coeliac disease, atrophic gastritis and acid-suppressing medication all impair absorption.

Supplementing without asking which of those applies fixes the number and leaves the cause running.

Repletion, and what to expect

Iron absorption is controlled by hepcidin, a liver hormone that blocks the export of iron from gut cells into the blood. Hepcidin rises for hours after an iron dose, which means a large dose taken every day partially blocks its own absorption — one of the reasons more frequent dosing is not straightforwardly better (Camaschella, Blood 2019). Hepcidin also rises with inflammation, which is why repletion goes slowly in anyone with an active inflammatory process.

Practically: take oral iron away from coffee, tea, calcium and antacids, all of which impair absorption, and with vitamin C, which improves it. Expect symptoms to improve before ferritin does, and expect ferritin to climb slowly — stores refill over months, so retesting before about eight to twelve weeks tells you little. Gastrointestinal side effects are the usual reason people quit; a different salt, lower dose or less frequent dosing usually solves it. Intravenous iron is a clinical decision, for poor absorbers or severe deficiency. For genuine overload the treatment is therapeutic phlebotomy, alongside avoiding iron supplements and alcohol.

Bottom line

Ferritin reports iron stores and inflammation with the same number, which is why it is so often misread. Low ferritin causes real symptoms — fatigue, hair shedding, restless legs, poor exercise tolerance — well before anaemia appears, and the mechanism is cofactor shortage in mitochondria, follicles and dopamine synthesis rather than oxygen delivery. High ferritin is usually metabolic or inflammatory, not haemochromatosis, and transferrin saturation is the test that tells them apart. In every case, the question worth answering is why the number is where it is, not just how to move it.

Educational content, not medical advice. Laboratory interpretation and any treatment decision are made by a licensed physician after individual evaluation. Individual results vary.