Iron deficiency anemia

What is iron deficiency anemia?

Iron deficiency anemia is a type of anemia caused by low levels of iron in the body. It is the most common cause of anemia among adults. Anemia means a decreased amount of hemoglobin in the blood. This can be due to a decrease in the number of RBCs in your blood or a decrease in the amount of hemoglobin in each RBC. Because the body uses oxygen to make energy, a person with anemia has less oxygen in their blood which can make them feel tired or short of breath.

Red blood cells

The blood contains many different types of cells. These cells include immune cells and red blood cells (RBCs). RBCs are responsible for carrying oxygen from your lungs to the rest of your body and carrying carbon dioxide back to the lungs. They hold on to oxygen and carbon dioxide using a specialized protein called hemoglobin.

Normal red blood cells

Red blood cells are made in a part of the bone called the bone marrow. As young RBCs mature in the bone marrow, they produce hemoglobin. Your body needs iron in order to make hemoglobin for RBCs. Once this process is complete, the RBCs are released into the bloodstream. Normal, healthy RBCs circulate in the bloodstream for about 120 days before they are removed, and their iron is recycled to make new RBCs.

Extra iron is stored in a specialized protein called ferritin. The amount of ferritin will change as the amount of iron in your body changes. For example, a person with low levels of iron in their body will have low levels of ferritin in their blood.

What causes iron deficiency anemia?

Under normal conditions, the body gets all of the iron it needs to make new RBCs from a combination of recycled iron (from RBCs after they die) and iron from the diet. The amount of iron in the body is carefully controlled. A specialized protein called transferrin is used to carry iron from recycled RBCs and absorbed in the small intestine to areas of the body where iron is needed.

If iron levels in the body are low, new iron can be released from food in the stomach and then absorbed in the intestine. Any condition that causes the body to lose RBCs or reduces the amount of new iron absorbed from food can lead to iron deficiency.

The most common reasons for developing iron deficiency are:

  • Blood loss – This can result from menstrual bleeding, bleeding from the gastrointestinal tract, or an injury that resulted in significant bleeding.
  • Pregnancy – Unborn babies receive iron from their mothers. For this reason, expecting mothers need extra iron in their diet to make up for this increased demand.
  • A diet low in iron – It is important to eat a diet that contains at least some iron-rich foods. Good sources of iron include meat, fish, and whole-grain cereals. Vitamin C can help increase iron absorption. Eating a diet low in iron will eventually cause anemia of iron deficiency.
  • Reduced absorption of iron in the gastrointestinal tract – Iron from food is absorbed primarily in the small intestine. Any condition that prevents the small intestine from working normally can decrease the amount of iron absorbed into the body. Conditions associated with reduced absorption are celiac disease, intestinal parasites, surgeries of the stomach or small intestine, inflammatory diseases of the intestine like Crohn’s disease.

When iron levels are low, the immature RBCs are unable to make enough hemoglobin. As a result, the new RBCs will be smaller (microcytic) and paler (hypochromic) than normal RBCs (see picture below). The cells also tend to vary in shape compared to normal RBCs that are more uniform. These changes take time to develop and may not be seen at the early stages of iron deficiency.

Iron deficiency anemia

How do doctors test for iron deficiency?

A blood test is the most common way to test for iron deficiency. The blood test will measure both the amount of iron in your blood and the proteins that regulate iron levels.

  • Ferritin – The level of ferritin in the blood provides a good estimate of the amount of iron stored in the body. Ferritin will be decreased in iron deficiency.
  • Iron – This measures the amount of iron in the blood. Although the amount of iron in the blood is decreased in iron deficiency, this test is not especially useful on its own because the levels vary a lot throughout the day and can be decreased for reasons other than iron deficiency.
  • Total iron-binding capacity (TIBC) – This measures how much transferrin protein is available to hold iron. In iron deficiency, less iron is attached to transferrin so the TIBC will be increased.
  • Transferrin saturation – This value represents the amount of iron in the blood divided by the TIBC, shown as a percentage. The transferrin saturation is decreased in iron deficiency.

Iron deficiency can also be diagnosed after a small sample of bone marrow is removed in a procedure called a bone marrow aspirate. The tissue sample is then examined under a microscope by a pathologist using special stains to look at the amount of iron in the tissue sample. Usually, this is not necessary to make the diagnosis since the blood tests can provide an answer.

by Rosemarie Tremblay-LeMay MD FRCPC, updated March 8, 2021
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