Ferritin
Iron-storage protein; the single best marker of body iron stores — but also an acute-phase reactant.
What it measures
Ferritin sequesters Fe(III) in a hollow protein shell (up to 4500 Fe atoms). Serum ferritin is mostly secreted by macrophages and hepatocytes in proportion to intracellular stores — except during inflammation, when synthesis is upregulated by IL-6.
Why we order it
Diagnose iron deficiency (most specific marker), screen for iron overload (HFE-related hemochromatosis, transfusional siderosis), and monitor therapy.
Interpretation by lens
In symptomatic patients with ferritin <30 µg/L, treat empirically with iron and investigate cause concurrently. Above 1000 µg/L, work up overload, inflammation, and liver disease.
Functional iron deficiency shows symptoms (fatigue, restless legs, exertional dyspnea) at ferritin 30–60 even without anemia; many functional clinicians treat to ≥70–100.
Ferritin glycation, isoform composition (H vs L chains), and intracellular ferritin localization are active research areas; serum ferritin is a rough proxy.
Causes of abnormal values
- ↑Inflammation / infection (acute-phase response)
- ↑Iron overload (hereditary hemochromatosis, transfusional)
- ↑Liver disease (release from hepatocytes)
- ↑Malignancy
- ↑Adult-onset Still's disease (extreme elevations >10 000)
- ↑Metabolic syndrome / NAFLD
- ↑Daily alcohol use
- ↓Iron deficiency (always — there is no false low)
- ↓Hypothyroidism (mild)
Pattern recognition
Iron deficiency. Investigate source: GI loss in adults of either sex, menstrual loss in premenopausal women.
Iron overload — HFE genotype, MRI T2*, consider phlebotomy.
Inflammation, not overload. Treat the cause; do not phlebotomize.
Pre-analytic & interpretation traps
- !Ferritin <30 µg/L is essentially diagnostic of iron deficiency; <100 in inflammation/heart failure can still represent deficiency.
- !BRITISH SOCIETY OF GASTROENTEROLOGY uses <30 µg/L; in chronic inflammation thresholds rise to <100 µg/L.
- !Heterophile antibodies and rheumatoid factor can falsely elevate immunoassay ferritin.
Logical next-step labs
What the data says
Primary literature
- British Society of Gastroenterology guidelines on iron deficiency — Snook J et al., Gut 2021