Ferritin

Ferritin is the body's main iron-storage protein. The amount of ferritin circulating in blood reflects how much iron is held in tissue stores (mostly liver, spleen and bone marrow). Because ferritin is also an acute-phase reactant, levels can rise temporarily during infection or inflammation, which can mask underlying iron deficiency.

Ferritin is the most useful single test for diagnosing iron deficiency in Australian adults. A low ferritin almost always indicates depleted iron stores, even when haemoglobin is still normal (non-anaemic iron deficiency). High ferritin can indicate iron overload (haemochromatosis), liver disease, chronic inflammation, or metabolic syndrome. Ferritin is part of standard iron studies and is also used to monitor iron infusion therapy and TRT-related red cell increases.

High ferritin has many possible causes and rarely points directly to iron overload alone. The most common Australian causes are metabolic syndrome and fatty liver disease (where chronic low-grade inflammation raises ferritin into the 300–800 μg/L range), excessive alcohol use, and any acute infection or inflammatory condition (ferritin is an acute-phase reactant — it can double during a chest infection without indicating iron problems). Genuinely high ferritin from iron overload — hereditary haemochromatosis — is the diagnosis of concern: persistently elevated ferritin with high transferrin saturation (>45%) warrants HFE gene testing. Other causes include liver disease, chronic kidney disease on dialysis, recent blood transfusions, certain cancers (lymphoma, leukaemia), and rare hereditary hyperferritinaemia syndromes. A single high ferritin should be repeated 4–6 weeks later when not unwell, ideally fasting.

Low ferritin (typically <30 μg/L by RACGP guidelines, though some specialists use <50 or <100 μg/L for symptomatic patients) almost always means depleted iron stores. The major causes are inadequate dietary iron (vegan, vegetarian and high-carbohydrate diets), heavy menstrual bleeding (a leading cause in premenopausal Australian women), pregnancy and lactation, gastrointestinal blood loss (peptic ulcer, NSAID use, coeliac disease, polyps, colorectal cancer), distance running and frequent blood donation. Importantly, "non-anaemic iron deficiency" — low ferritin with normal haemoglobin — is increasingly recognised as a cause of fatigue, hair loss, restless legs and exercise intolerance and is often treated with oral iron or, in symptomatic cases unresponsive to oral iron, an iron infusion (Ferinject is widely used in Australia).

Ferritin should be tested at least annually in premenopausal women, vegans, vegetarians, distance runners, frequent blood donors, pregnant or breastfeeding women, and anyone with chronic GI conditions. People being treated for iron deficiency should retest 4–6 weeks after starting oral iron, then every 3 months until ferritin reaches the upper-normal range (RACGP suggests aiming for >100 μg/L in symptomatic patients), then annually. Men over 40 with metabolic risk factors should test ferritin and transferrin saturation periodically to screen for haemochromatosis.

Interpreted alongside the rest of the iron studies panel (serum iron, transferrin / TIBC, transferrin saturation), plus haemoglobin, MCV and MCH from the FBC. CRP is often added when ferritin is high to rule out inflammation as the cause.