Ferritin is the protein that stores iron inside cells, and a blood ferritin level is the best single marker of the body's iron stores. A low ferritin means those stores are depleted — iron deficiency — which can cause symptoms even before it tips into anemia. See what low ferritin means.
One important caveat: ferritin is also an acute-phase (inflammation) marker (see inflammation), so infection, inflammation or liver problems can push it falsely normal or high and mask a deficiency. Results should be interpreted with symptoms by a clinician.