A hemoglobin of 66 g/L below normal is considered moderate anemia and the cause needs to be identified and the anemia corrected as soon as possible. Common causes of lowered hemoglobin include iron deficiency anemia, malnutrition, long-term infections, other blood system diseases and immune system diseases. 1. Iron deficiency anemia: it is mostly caused by iron deficiency in food or chronic bleeding disorders. Iron is the main raw material for hemoglobin synthesis, and if iron intake is insufficient, it will affect the hemoglobin content, so hemoglobin will drop and lead to anemia. 2. Malnutrition: In addition to iron, vitamin B₁₂, folic acid and other nutrient deficiencies will also affect the amount of hemoglobin, resulting in anemia, especially in pregnant women. 3. Long-term infections: Long-term infections such as intestinal infections and Crohn’s disease can also lead to a decrease in hemoglobin count and anemia symptoms. 4. Other blood disorders and immune system diseases: such as aplastic anemia, acute leukemia, systemic lupus erythematosus, etc., can also have a decrease in hemoglobin. Reduced hemoglobin may also have other causes, such as tumors, etc. If there is a drop in hemoglobin, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, and then under the guidance of the physician to give targeted treatment or therapy.