Iron Deficiency

Iron is necessary for the production and function of hemoglobin and myoglobin. It also plays critical roles in mitochondrial energy production and neurotransmitter synthesis. The recommended daily intake of iron is 18 mg per day for women, but the average intake in the United States is approximately 13 mg/day. Iron deficiency affects approximately 2 billion people worldwide and 14% of adults in the United States. 

Iron is absorbed from the diet. The most efficiently absorbed form is heme iron, derived from red meat, poultry, and seafood. Nonheme iron is available from legumes and vegetables (dried beans, dark leafy greens) and supplemented cereals. Both forms of iron are absorbed by intestinal enterocytes and transported via the iron exporter protein, ferroportin, to transferrin in the blood. Transferrin transports iron to the liver and spleen for storage as ferritin. Transferrin also transports iron to the bone marrow where it is stored as  hemosiderin. Iron is also salvaged from senescent red blood cells by macrophages and stored in the reticuloendothelial system. Approximately 1 to 2 mg of iron is lost each day in sweat and feces.

Hepcidin is a hormone synthesized predominantly in the liver that regulates iron absorption and storage. Hepcidin blocks ferroportin from absorbing iron from the GI tract and prevents the release of stored iron from the liver, spleen, and bone marrow. Hepcidin decreases when iron stores are low, increasing iron absorption and the release of stored iron. Conversely, hepcidin increases when iron stores are adequate, suppressing iron absorption and release from storage. Hepcidin also increases during inflammation. 

The most common causes of iron deficiency are bleeding (menstrual and gastrointestinal), impaired iron absorption (atrophic gastritis, celiac disese, bariatric surgery), inadequate dietary intake, and pregnancy. During the third trimester of pregnancy, up to 84% of women develop iron deficiency. Additional risk factors include use of nonsteroidal anti-inflammatory drugs, inflammatory bowel disease, chronic kidney disease, and cancer. 

Iron deficiency progresses from low iron stores to iron deficiency anemia. Absolute iron deficiency is defined as serum ferritin less than 30 ng/dL or transferrin saturation (TSAT) less than 16%-20%. TSAT is calculated as iron/transferrin x 1.2. Iron deficiency anemia is defined as ferritin less than 30 ng/mL, or TSAT less than 20%, and a hemoglobin level less than 12 g/dL in women and less than 13 g/dL in men. 

Individuals with iron deficiency or iron-deficiency anemia may be asymptomatic or experience symptoms such as fatigue, irritability, depression, difficulty concentrating, restless legs syndrome, pica, dyspnea, lightheadedness, exercise intolerance, and worsening heart failure.

All patients with symptoms of iron deficiency, anemia, or microcytosis should undergo testing for complete blood count (CBC), ferritin, and/or TSAT. Serum ferritin is the preferred test for iron deficiency because iron deficiency is the only cause of a low level. 

Serum ferritin originates primarily from excess iron stored in macrophages that is not used for hemoglobin synthesis. The amount of ferritin in plasma directly reflects the total body iron stored as ferritin in tissues. A serum ferritin less than 15 mg/L is 99% specific for making a diagnosis of iron deficiency.  

Ferritin is less reliable in diagnosing iron deficiency in patients with inflammatory conditions because it is an acute phase reactant and is elevated. These disorders may mask iron deficiency.  In these situations, a ferritin level of less than 50 ng/mL is consistent with iron deficiency. If a patient is suspected of having iron deficiency and has a ferritin level of greater than 50 mg/dL, then TSAT should be measured. 

TSAT determines the percentage of transferrin that is saturated with iron. Normally, transferrin is only 25-30% saturated with iron. Both iron concentration and transferrin saturation decrease with iron deficiency. Transferrin saturation less than 16% indicates iron deficiency. A higher threshold of 20% is often used for patients with inflammation. TSAT should not be measured within 5 to 9 hours of ingesting iron-containing foods, vitamins, or supplements. 

Hemoglobin levels and mean cell volume (MCV) are not very sensitive indicators of iron deficiency because anemia and microcytosis do not occur until the later stages of iron deficiency anemia.

Serum iron, and therefore transferrin saturation, exhibit biological variation due to diurnal variation and post-prandial effects. Levels can vary by 25-30% from day to day.  

The reference range for transferrin saturation is 15-50%. 

The reference range for ferritin is 20-200 ng/mL in women and 20-300 ng/mL in men.

Specimen requirement for both tests is a red top tube of blood.

References

Auerbach M et al. Iron Deficiency in Adults: A Review. JAMA, published online March 30, 2025,  doi:10.1001/jama.2025.0452.

Camaschella C. Iron deficiency. Blood. 2019;133(1):30-39. 

Guyatt GH, Oxman AD, Ali M, Willan A, McIlroy W, Patterson C. Laboratory diagnosis of iron- deficiency anemia: an overview. J Gen Intern Med. 1992;7(2):145-153. 

Markowitz H, Fairbanks VF: Transferrin assay and total iron binding capacity. Mayo Clin Proc 1983;58:827-828

Szoke D, Panteghini M: Diagnostic value of transferrin. Clin Chim Acta 2012 Aug 16;413(15-16):1184-1189