Ferritin is the intracellular protein that stores iron in a non-toxic, bioavailable form. Serum ferritin reflects body iron stores.
Mechanism — Iron deficiency drops ferritin first (before hemoglobin). But ferritin is ALSO an acute-phase reactant — elevated in inflammation, infection, liver disease, and metabolic syndrome. Always interpret in context.
Lab range — Men: 30-400 ng/mL. Women: 15-150 ng/mL. Optimal range for symptoms typically 50-150 ng/mL. Above 300 ng/mL flag for hemochromatosis or inflammation.
Caveats — Low ferritin with normal hemoglobin still produces fatigue and exercise intolerance — symptoms appear before anemia. High ferritin without a clear acute cause warrants iron studies and HFE testing.