Losing 100 hairs a day is normal but if you’re seeing a lot more than that every time you brush your hair, this may be a sign you lack iron. Iron deficiency can lead to anemia. When that happens, the hair follicles don’t get enough oxygen, which the body uses just for vital functions because it’s in survival mode, and the hair falls out. Treatment for hair loss is enhanced when iron deficiency, with or without anemia, is treated, according to a review of several studies.