Stores empty before the blood count moves
Iron deficiency develops in stages rather than all at once. First the body draws down its reserves, and the storage protein ferritin falls; a low ferritin is often the earliest laboratory sign, while a routine blood count can still look normal. Only after stores are depleted does the deficiency progress.
This staged pattern is why someone can feel symptoms or have low ferritin well before anemia appears on standard testing.
From transport shortfall to anemia
In the second stage, iron available for transport drops: transferrin saturation falls as the carrier protein finds less iron to carry. With too little iron reaching the bone marrow, red blood cell production finally suffers. The result is iron-deficiency anemia, the third stage, typically with small, pale red cells and reduced hemoglobin.
This is general educational information, not medical advice. Because anemia can have several causes, persistent fatigue or abnormal blood results should be evaluated by a clinician rather than treated with iron on assumption, as excess iron is harmful.