Iron is an essential trace element and nutrient that is necessary to maintain life. The serum iron test measures the amount of iron in the liquid portion of blood.
Iron is absorbed from food and transported throughout the body by transferrin, a protein produced by the liver. Iron is necessary for the production of healthy red blood cells (RBCs). It is an important part of hemoglobin, the protein in RBCs that enables them to carry oxygen throughout the body. Iron is also used in the production of some proteins, including myoglobin and some enzymes.
Normally, about 70% of the iron absorbed is incorporated into the production of hemoglobin inside RBCs. The remainder is stored in the tissues as ferritin or hemosiderin. If not enough iron is taken in from the diet, blood levels will drop; thus, over time, the iron stored in the tissues will be used, eventually depleting the stored iron. Insufficient levels of circulating iron and iron storage will eventually lead to iron deficiency anemia. On the other hand, absorption of too much iron can lead to progressive accumulation and damage to organs such as the liver, heart, and pancreas.
The serum iron test measures the amount of iron that is in transit in the body – the iron that is bound to transferrin. The amount of iron present in the blood will vary throughout the day and from day to day. For this reason, serum iron is almost always measured with other iron tests, such as serum ferritin and the total iron-binding capacity (TIBC), from which the transferrin saturation can be calculated. Transferrin saturation reflects the amount of iron being transported in the blood and its capacity to carry more. The use of several iron tests provides a more reliable measure of iron deficiency and iron overload than measuring serum iron by itself.