The blood transferrin level is tested for diverse reasons: to determine the cause of anemia, to examine iron metabolism (for example, in iron deficiency anemia) and to determine the iron-carrying capacity of the blood.
Low transferrin can impair hemoglobin production (since to make hemoglobin, you have to have iron) and so lead to anemia. Low transferrin can be due to poor production of transferrin by the liver (where it's made) or excessive loss of transferrin through the kidneys into the urine. Many conditions including infection and malignancy can depress transferrin levels. The transferrin is abnormally high in iron deficiency anemia.
Hereditary absence of transferrin is called atransferrinemia. It is characterized by anemia and hemosiderosis (iron deposition) in the heart and liver. The iron damage to the heart can lead to heart failure. The anemia is typically microcytic and hypochromic (the red blood cells are abnormally small and pale). The disease is inherited as an autosomal recessive trait. It is due to mutation of both of a person's transferrin genes. Atransferrinemia can be effectively treated by plasma infusions of transferrin.