Low ferritin is the result of iron deficiency.
Ferritin is the storage form of iron in the body, and when there is an iron deficiency in the body, ferritin decreases, and the degree of ferritin decrease correlates with the degree of iron deficiency.
Usually, iron deficiency up to a certain level leads to a decrease in ferritin, which in turn leads to the development of iron deficiency anemia. Reduced ferritin is also a diagnostic indicator of iron deficiency anemia.
Iron deficiency anemia requires iron supplementation, both to normalize ferritin and to find and remove the cause of the deficiency.
If the cause of iron deficiency cannot be removed, even if ferritin is normalized, iron supplementation is still needed, otherwise ferritin will drop again over time, leading to the development of iron deficiency anemia again.