Iron deficiency anemia is the most common type of anemia in clinical practice, and there are three stages in the sequence of changes in iron deficiency anemia. The first one, store iron deficiency. Due to the imbalance between the supply and demand of iron, it causes the lack of stored iron in the body. The patient does not have symptoms of anemia, and at the same time, the results of all routine blood tests are normal, but only the stored iron is reduced. The second one is iron deficiency erythropoiesis. The lack of stored iron continues to develop and causes iron deficiency in the red blood cells. The hemoglobin concentration of routine blood tests is usually normal, but the average volume of red blood cells and the average amount of hemoglobin in red blood cells can be reduced. Thirdly, iron deficiency anemia. Patients show symptoms of anemia and a decrease in hemoglobin, mean red blood cell volume and mean hemoglobin volume during routine blood tests. The causes of iron deficiency anemia are threefold: insufficient iron intake, impaired iron absorption, and chronic blood loss.