As your iron level increases, the liver produces molecules called Hepcidins that blocks Ferroportins (iron transporter in the intestines and macrophages). Blocking these transporters allows iron to be excreted, and for macrophages, stays within its storage. When there is a decrease in iron, hepcidin decreases, which allows for more absorption of iron and macrophages sends its iron to the bone marrow to build more red blood cells. Interesting fact, Vitamin C is an inhibitor of Hepcidin, which makes the reason why those with Scurvy (low Vitamin C) are usually anemic and have improper collagen formation.
