Soluble Transferrin Receptor
Also known as: sTfR
What Does Soluble Transferrin Receptor Measure?
Soluble Transferrin Receptor (sTfR) measures the concentration of a truncated form of the transferrin receptor that is shed from the surface of cells into the bloodstream. Transferrin receptors are proteins found on the surface of cells—particularly developing red blood cells in the bone marrow—that bind to transferrin (the iron-carrying protein) to facilitate iron uptake into cells. When cells are iron-deficient or when red blood cell production increases, more transferrin receptors are expressed on cell surfaces, leading to higher levels of the soluble form in the blood.
Why Does Soluble Transferrin Receptor Matter?
sTfR is a highly valuable clinical marker for diagnosing and distinguishing different types of anemia, particularly iron deficiency anemia versus anemia of chronic disease (also called anemia of inflammation). Unlike ferritin and other iron markers, sTfR levels are not significantly affected by inflammation, infection, or chronic illness, making it a more reliable indicator of true iron stores in people with co-existing inflammatory conditions. Elevated sTfR levels reliably indicate true iron deficiency at the tissue level, even when other markers like ferritin may be falsely elevated due to inflammation. It is also used to monitor iron status during pregnancy, assess erythropoietic activity, and evaluate response to iron replacement therapy.
Normal Ranges
Males
0.83–1.76 mg/L (may vary by assay; some labs report 1.9–4.4 mg/L using different reference methods)
Females
0.83–1.76 mg/L (non-pregnant; values are assay-dependent)
Children
Varies by age; generally higher in infants and young children due to rapid growth and higher erythropoietic activity
Causes of High Levels
- Iron deficiency anemia — the most common cause, as iron-starved cells upregulate transferrin receptor expression to capture more iron
- Hemolytic anemias (e.g., sickle cell disease, thalassemia) — increased red blood cell turnover drives higher erythropoietic activity
- Pregnancy — especially in the second and third trimesters when iron demand increases substantially
- Erythropoietin therapy — stimulates red blood cell production, increasing transferrin receptor expression in bone marrow
- Polycythemia vera — abnormally high red blood cell production raises sTfR levels
- Inadequate dietary iron intake or chronic malabsorption syndromes such as celiac disease or Crohn's disease
Causes of Low Levels
- Aplastic anemia — reduced bone marrow activity and red blood cell production decreases transferrin receptor expression
- Iron overload states (e.g., hereditary hemochromatosis) — cells reduce receptor expression when iron is abundant
How to Improve Your Soluble Transferrin Receptor
Diet
- Increase heme iron intake through red meat (beef, lamb), poultry, and seafood (clams, oysters), which is more bioavailable than plant-based iron
- Consume non-heme iron sources such as lentils, spinach, tofu, fortified cereals, and pumpkin seeds daily
- Pair iron-rich foods with vitamin C sources (citrus fruit, bell peppers, strawberries) to enhance non-heme iron absorption by up to 3-fold
- Avoid consuming coffee, tea, calcium-rich foods, or high-phytate foods (bran, raw legumes) within 1–2 hours of iron-rich meals, as these inhibit iron absorption
- Consider cooking in cast iron cookware, which can incrementally increase dietary iron content of acidic foods
Supplements
- Ferrous sulfate 325 mg (contains ~65 mg elemental iron) taken 1–3 times daily on an empty stomach or with vitamin C for maximum absorption — the most common and cost-effective iron supplement
- Ferrous gluconate 300 mg (contains ~35 mg elemental iron) as a gentler alternative for those experiencing gastrointestinal side effects from ferrous sulfate
- Ferrous bisglycinate chelate 25–50 mg elemental iron daily — a highly bioavailable form with fewer GI side effects, suitable for sensitive individuals
Related Biomarkers
Frequently Asked Questions
What is the difference between sTfR and ferritin for diagnosing iron deficiency?
Ferritin reflects iron storage levels but is an acute-phase protein, meaning it rises during inflammation, infection, or chronic disease—sometimes masking true iron deficiency. sTfR, on the other hand, reflects functional iron demand at the cellular level and is not influenced by inflammation. This makes sTfR particularly useful in patients with chronic conditions such as rheumatoid arthritis, inflammatory bowel disease, or chronic kidney disease, where ferritin may be misleadingly normal or elevated even when true iron deficiency exists. Using both markers together—or calculating the sTfR/log ferritin ratio—provides the most accurate picture of iron status.
Can sTfR distinguish iron deficiency anemia from anemia of chronic disease?
Yes, this is one of the most important clinical uses of sTfR. In iron deficiency anemia, sTfR is typically elevated because cells are actively trying to capture more iron. In pure anemia of chronic disease, sTfR is usually normal or low because erythropoiesis is suppressed and there is no true iron deficiency at the cellular level. When both conditions coexist—iron deficiency anemia occurring on top of a chronic disease—sTfR will be elevated, helping clinicians identify patients who need iron supplementation even when ferritin is normal or high due to inflammation.
Is sTfR testing affected by inflammation or infection?
No—this is one of the key advantages of sTfR over other iron markers. Unlike ferritin, C-reactive protein (CRP), or serum iron, sTfR concentrations are not significantly altered by acute or chronic inflammation, liver disease, or infection. This makes it a more reliable and stable marker of iron status across a wide range of clinical conditions, particularly in hospitalized patients or those with ongoing inflammatory diseases.