What is Ferritin and What Does Low Ferritin Mean?
Ferritin is a protein that stores iron in your body, and a low ferritin level is the most specific and earliest indicator of depleted iron stores, signaling iron deficiency before anemia develops.
Understanding Ferritin
Ferritin exists primarily inside cells throughout your body, with only a small amount circulating in your blood. Under normal conditions, serum ferritin concentration directly reflects the amount of iron stored in your body—approximately 1 μg/L of serum ferritin equals about 10 mg of stored iron 1.
Normal Ferritin Ranges
Average serum ferritin concentrations vary by sex and age 1:
- Men: 135 μg/L
- Women: 43 μg/L
- Children (6-24 months): approximately 30 μg/L
What Low Ferritin Means
Low ferritin indicates depleted iron stores and is diagnostic of iron deficiency. This is critically important because ferritin drops before anemia develops, making it an early warning sign 1.
Diagnostic Thresholds for Iron Deficiency
The evidence shows varying thresholds depending on population and clinical context:
For women of childbearing age:
- ≤15 μg/L confirms iron deficiency with 75% sensitivity and 98% specificity 1
- <12 μg/L is diagnostic with 61% sensitivity but 100% specificity 2
For general populations:
- <15 μg/L is highly specific (99% specificity) for iron deficiency 3
- <30 μg/L generally indicates low body iron stores 3
Recent evidence suggests higher thresholds may be more physiologically appropriate:
- Studies using physiological indicators (hemoglobin and transferrin receptor) found optimal thresholds of approximately 25 μg/L for women and 22 μg/L for children 4
- The 2026 American Society of Hematology guidelines recommend ≤20 μg/L as the threshold for diagnosing iron deficiency 5
Clinical Significance
Iron deficiency progresses through stages:
- Depleted iron stores (low ferritin, normal hemoglobin)
- Iron-deficient erythropoiesis (insufficient iron for red blood cell production)
- Iron deficiency anemia (low hemoglobin)
Low ferritin catches the problem at stage 1, before functional consequences develop 1.
Critical Caveat: The Inflammation Problem
Ferritin is an acute-phase reactant, meaning it rises during inflammation, infection, or tissue damage independent of iron status 1. This can mask true iron deficiency.
When Ferritin May Be Falsely Normal or Elevated
Ferritin can be elevated despite iron deficiency in:
- Chronic infections
- Inflammatory conditions (arthritis, inflammatory bowel disease)
- Liver disease (hepatitis, cirrhosis)
- Malignancy
- Chronic kidney disease 1, 6
In these situations:
- Ferritin <100 μg/L may still indicate iron deficiency despite being "normal" 6
- Ferritin >150 μg/L makes absolute iron deficiency unlikely even with inflammation 3
- Consider transferrin saturation <20% as additional evidence of iron deficiency 6
Practical Approach When Inflammation Is Present
For patients with chronic inflammatory conditions and ferritin 15-100 μg/L:
- Use ferritin <45 μg/L as a threshold (provides 92% specificity while maintaining reasonable sensitivity) 3
- Consider additional testing: transferrin saturation, soluble transferrin receptor, or percentage of hypochromic red cells 6
- A trial of iron supplementation may be diagnostic if uncertainty remains 6
Clinical Implications of Low Ferritin
Even without anemia, iron deficiency causes:
- Fatigue and reduced exercise capacity
- Impaired cognitive function
- Hair loss 7
- Reduced work productivity
- In pregnancy: increased risk of preterm delivery and low birth weight
The key message: Don't wait for anemia to develop. Low ferritin alone warrants investigation and treatment, particularly in men, postmenopausal women, and those with gastrointestinal symptoms, as it may indicate occult bleeding from serious conditions including gastrointestinal malignancy 2, 3.