Ferritin is the major iron storage protein in the body, found predominantly in the liver, spleen, and bone marrow. A small amount circulates in the blood and serves as the most accurate single measure of total body iron stores. Unlike serum iron, which fluctuates with recent meals and daily variation, ferritin reflects the overall iron reserve and is a stable, reliable marker.

Ferritin is low in iron deficiency and iron deficiency anaemia. It is elevated when iron stores are excess (haemochromatosis, iron overload) and also rises non-specifically as an acute phase reactant in inflammation, infection, liver disease, and malignancy. This means ferritin must be interpreted alongside other iron markers (serum iron, transferrin saturation) and clinical context to distinguish iron overload from an inflammatory elevation.