Lab Reference Library / Soluble Transferrin Receptor (sTfR)
Advanced & Specialty
Soluble Transferrin Receptor (sTfR)
sTfR · Soluble Transferrin Receptor · Serum Transferrin ReceptorReference range, optimal functional medicine levels, and why sTfR is the most accurate iron deficiency marker that remains unaffected by inflammation, why it outperforms ferritin for identifying true iron deficiency in patients with inflammatory conditions, and how the sTfR/log ferritin index distinguishes iron deficiency from anemia of chronic disease.
Iron Deficiency MarkerSpecialty Testing
Reference Range0.83 to 1.76 mg/L
ElevatedIron Deficiency
Normal in ACDTrue Benefit
Unitsmg/L
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Category: Advanced & Specialty | Also known as: sTfR, Transferrin Receptor, sTfR/log Ferritin Index
1. What This Test Measures
The soluble transferrin receptor (sTfR) is a truncated form of the membrane-bound transferrin receptor (TfR1) that is cleaved and released into the circulation in proportion to the number of transferrin receptors expressed on cell surfaces. TfR1 expression is tightly regulated by intracellular iron levels: when intracellular iron is low, cells upregulate TfR1 expression to capture more transferrin-bound iron from the circulation; when iron is replete, TfR1 expression is downregulated. Because sTfR in the blood is a byproduct of TfR1 shedding, its concentration reflects the aggregate iron demand of all iron-requiring cells, with erythroid precursors in the bone marrow being the dominant contributors given their intense iron requirements for hemoglobin synthesis.
The critical clinical advantage of sTfR over ferritin is its independence from inflammatory status. Ferritin is an acute phase reactant that rises with infection, inflammation, metabolic syndrome, obesity, and liver disease regardless of actual iron stores. A patient with true iron deficiency and concurrent rheumatoid arthritis, chronic kidney disease, or obesity may have a ferritin of 80 to 120 ng/mL, within or even above the standard reference range, while actually being functionally iron deficient at the cellular level. sTfR rises reliably with iron deficiency regardless of inflammatory burden, making it the diagnostic marker of choice in these challenging clinical contexts. The sTfR/log ferritin index combines both markers to produce a single value that differentiates true iron deficiency from anemia of chronic disease with high accuracy.
2. Reference Ranges and Clinical Interpretation
| sTfR (mg/L) | Interpretation |
|---|---|
| 0.8 to 1.76 mg/L | Normal: iron-deficient erythropoiesis unlikely; iron stores adequate at tissue level |
| 1.76 to 2.5 mg/L | Borderline elevated: mild iron-deficient erythropoiesis; interpret alongside ferritin, TIBC, iron saturation |
| Above 2.5 mg/L | Elevated: significant iron-deficient erythropoiesis; investigate for absolute or functional iron deficiency |
| sTfR/log Ferritin above 2.0 | Iron-deficient erythropoiesis confirmed; supplement or treat underlying cause |
| sTfR/log Ferritin below 1.0 | Anemia of chronic disease; iron stores adequate; targeting the underlying inflammatory process is the treatment |
Reference ranges vary by laboratory and assay. sTfR is elevated in hemolytic anemias, polycythemia, and thalassemia through mechanisms unrelated to iron deficiency; always interpret with CBC and reticulocyte count. The sTfR/log ferritin index requires both values from the same blood draw.
3. Why Ferritin Alone Fails in Inflammatory Conditions
Ferritin serves dual roles in the body: as an iron storage protein within cells and as an acute phase reactant whose production is directly induced by inflammatory cytokines, particularly IL-1, IL-6, and TNF-alpha. During any inflammatory state, these cytokines stimulate hepatic ferritin synthesis independently of iron stores, artificially elevating serum ferritin. The clinical consequence is that standard iron deficiency criteria (ferritin below 20 to 30 ng/mL) fail to detect iron deficiency in a large proportion of patients with concurrent inflammation.
Conditions That Falsely Elevate Ferritin
- Chronic inflammatory diseases (RA, IBD, lupus)
- Chronic kidney disease
- Obesity and metabolic syndrome
- Non-alcoholic fatty liver disease
- Active infection or sepsis
- Malignancy (ferritin is an acute phase reactant in cancer)
- Hypothyroidism (slowed ferritin clearance)
When sTfR Is the Superior Test
- Fatigue, hair loss, restless legs, or cold intolerance with ferritin in the 30 to 150 range and elevated CRP
- Anemia workup in patients with known chronic inflammatory, autoimmune, or renal disease
- Pre-operative anemia evaluation where inflammation is present
- Monitoring iron deficiency treatment response when ferritin is unreliable
- Athletes with elevated ferritin from training inflammation
4. Distinguishing Iron Deficiency from Anemia of Chronic Disease
One of the most common and clinically consequential diagnostic challenges in internal medicine and functional medicine is distinguishing true iron deficiency anemia from anemia of chronic disease (ACD). ACD results from inflammatory cytokine-driven suppression of erythropoiesis and iron utilization rather than absolute iron depletion. The treatment differs fundamentally: iron deficiency anemia requires iron supplementation or treatment of the bleeding source, while ACD requires treatment of the underlying inflammatory disease. Giving iron supplementation to a patient with pure ACD does not improve the anemia and may worsen the underlying inflammatory process by providing iron for pathogen growth or oxidative damage.
The sTfR/log ferritin index distinguishes these conditions with high sensitivity and specificity. In true iron deficiency, sTfR is elevated (high iron demand) and ferritin is low, producing an index above 2. In ACD with adequate iron stores, sTfR is normal to low (no increased iron demand at the erythroid level) and ferritin is elevated by inflammation, producing an index below 1. In ACD with superimposed true iron deficiency (a common mixed picture in chronic disease), both sTfR and ferritin are elevated and the index falls in the intermediate range, signaling the need to address both the inflammation and the iron deficit.
5. Related Lab Tests
FerritinIron storage; complement sTfR for complete iron status picture
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TIBC / TransferrinIron binding capacity; elevated in true iron deficiency
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Iron SaturationFunctional iron availability; below 20% suggests iron-deficient erythropoiesis
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hs-CRPInflammation level determines ferritin reliability
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6. Clinical Perspective
Clinical Perspective
The patient who changed how I use sTfR was a 44-year-old woman with Hashimoto thyroiditis, chronic fatigue, restless legs, and hair loss whose ferritin was 68 ng/mL. Her prior physicians had looked at that ferritin and told her she was fine. Her sTfR was 3.1 mg/L and her sTfR/log ferritin index was 2.8: she had significant iron-deficient erythropoiesis despite an apparently normal ferritin. Her inflammatory state from Hashimoto disease had inflated her ferritin while her tissues were starving for iron. Once we addressed both the autoimmune inflammation and the functional iron deficiency with IV iron followed by oral maintenance, her fatigue, hair loss, and restless legs resolved over three months. Ferritin alone would have sent her home with nothing. sTfR gave us the diagnosis and the treatment target.
Brian Lamkin, DO | Founder, The Lamkin Clinic | Edmond, Oklahoma
7. Frequently Asked Questions
Why is sTfR more useful than ferritin in inflammation?
Ferritin is an acute phase reactant that rises with inflammation, infection, obesity, and liver disease regardless of iron stores. A patient with true iron deficiency and concurrent inflammation may have a falsely normal or elevated ferritin. sTfR rises with iron deficiency regardless of inflammatory status, making it the preferred iron deficiency marker in patients with chronic inflammatory conditions.
What is the sTfR/log ferritin index?
The sTfR/log ferritin index divides the sTfR value by the log of the ferritin concentration. Values above 2 indicate iron-deficient erythropoiesis (true iron deficiency), while values below 1 suggest anemia of chronic disease with adequate iron stores. This index is particularly useful for distinguishing the two in patients with elevated inflammatory markers.
When should sTfR be ordered?
Order sTfR when ferritin is borderline or elevated in a patient with symptoms of iron deficiency (fatigue, cold intolerance, restless legs, hair loss) and elevated inflammatory markers (CRP, ESR). Also useful in patients with autoimmune conditions, chronic kidney disease, cancer, or obesity where ferritin is chronically elevated due to inflammation.
Does hemolysis or polycythemia affect sTfR?
Yes. Hemolytic anemias, polycythemia vera, and thalassemia trait all increase erythropoietic activity and elevate sTfR through iron demand independent of iron stores. Interpret sTfR in context of the full CBC, reticulocyte count, and peripheral smear when these conditions are possible.
Can sTfR be used to monitor iron repletion?
Yes. As iron stores replenish and iron-deficient erythropoiesis resolves, sTfR falls toward normal. This makes it a useful marker for monitoring treatment response, particularly in patients where ferritin remains unreliable due to persistent inflammation. A falling sTfR alongside improving symptoms confirms adequate iron repletion at the tissue level.
Content authored and clinically reviewed by Brian Lamkin, DO, founder of The Lamkin Clinic in Edmond, Oklahoma. Brian Lamkin, DO has 25+ years of experience in functional and regenerative medicine. This page reflects current functional medicine practice standards and is updated as new clinical evidence becomes available.
Evaluate Iron Deficiency at the Tissue Level
Soluble transferrin receptor identifies functional iron deficiency when ferritin is falsely elevated by inflammation. Contact us to discuss your complete iron status.
Schedule a ConsultationMedical Disclaimer: This content is provided for educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Lab interpretation should always be performed in clinical context by a qualified healthcare provider. Reference ranges and optimal targets may vary based on individual patient history, clinical presentation, and laboratory methodology. Schedule a consultation to discuss your specific results with Dr. Lamkin.