Ferritin is the body’s main iron storage protein and the most useful single marker of iron stores. Typical reference ranges are about 30–300 ng/mL for men and 15–150 ng/mL for women, though labs vary. Low ferritin is highly suggestive of depleted iron stores and is one of the clearest markers of iron deficiency. High ferritin is less specific because it also rises with inflammation, liver disease, alcohol use, metabolic dysfunction, and iron overload. In healthy individuals, ferritin is most informative as a marker of iron reserve, but elevated values need context.