Iron deficiency typically progresses in three stages: (1) Depleted iron stores—ferritin drops while hemoglobin may remain normal. (2) Functional deficiency—iron delivery and enzyme-dependent processes are affected and symptoms may begin. (3) Iron-deficiency anemia—hemoglobin falls and classic anemia signs become clear. Progression speed varies. Catching it early is easier to correct, which is why ferritin monitoring matters.