What causes microcytosis?

Low Fe. Microcytic rbcs can be caused by a number of things but, by far, the most common cause is iron deficiency. This can be caused by a low dietary intake of iron, poor absorption of iron for some reason, or blood loss. In pre-menopausal, post-pubertal women, the most common cause is blood loss from monthly menstrual flow. Low iron intake exacerbates this.