Why is my ferritin level low when my serum iron level is normal?

Understanding Low Ferritin with Elevated Serum Iron

Your ferritin of 12 μg/L indicates depleted iron stores (absolute iron deficiency), while your serum iron of 216 μg/dL represents only the iron currently circulating in your bloodstream—these measure completely different aspects of iron status and can be discordant. 1

Why These Values Don't Contradict Each Other

Serum iron and ferritin measure fundamentally different things:

• Serum iron measures iron bound to transferrin that is currently in transit through your bloodstream, available for immediate use in red blood cell production 1
• Ferritin reflects the total iron stored in your liver, spleen, and bone marrow—your body's iron reserves 2, 1
• These parameters should always be interpreted together, as neither alone provides a complete picture of iron status 1

Your serum iron can be temporarily normal or even elevated despite depleted stores because:

• Serum iron shows significant diurnal (time-of-day) variation, fluctuating throughout the day 2
• A single elevated serum iron measurement may simply reflect the timing of your blood draw 2
• Your body can mobilize iron from tissues into circulation even when stores are critically low 3

Confirming True Iron Deficiency

Your ferritin of 12 μg/L has 99% specificity for absolute iron deficiency—this is diagnostic. 1, 4

To complete the assessment, calculate your transferrin saturation (TSAT):

• TSAT = (serum iron × 100) ÷ total iron-binding capacity (TIBC) 2, 1
• TSAT <16% confirms impaired iron delivery to bone marrow for red blood cell production 2
• TSAT <20% is the traditional threshold indicating iron deficiency in most clinical contexts 2, 1

Request these additional tests if not already done:

• Complete blood count (hemoglobin/hematocrit) to assess for anemia 2
• TIBC or transferrin to calculate TSAT 2, 1
• C-reactive protein (CRP) to rule out inflammation that could confound interpretation 2, 1

Critical Caveat: Rule Out Inflammation

Ferritin is an acute-phase reactant that rises during inflammation, infection, or tissue damage—but your ferritin is extremely low, making this unlikely. 1, 5

• If CRP is elevated, iron deficiency may still be present with ferritin levels up to 100 μg/L in inflammatory conditions 2, 1
• Your ferritin of 12 μg/L is well below even the inflammatory threshold, confirming depleted stores regardless 1

Clinical Significance and Next Steps

Even without anemia, your depleted iron stores (ferritin 12 μg/L) cause significant symptoms:

• Fatigue and lethargy are common with low ferritin alone, before anemia develops 1
• Reduced aerobic performance and exercise intolerance occur with iron deficiency 1
• This represents Stage 1 iron deficiency, where stores are depleted but hemoglobin may still be normal 1

Immediate management algorithm:

• Initiate oral iron supplementation immediately (ferrous sulfate 325 mg daily or equivalent) 1
• Investigate the source of iron loss: gastrointestinal blood loss, heavy menstrual bleeding, dietary insufficiency, malabsorption 1
• Recheck ferritin in 8-12 weeks to confirm response to supplementation 2
• Consider intravenous iron if oral iron is not tolerated or if there is malabsorption 2

Common Pitfall to Avoid

Do not be falsely reassured by a normal or elevated serum iron value when ferritin is low. 3

• Serum iron reflects only circulating iron at that moment, not total body stores 1
• Ferritin <15 μg/L is the earliest and most specific marker of iron deficiency, appearing before other parameters become abnormal 1, 4
• The correlation between serum iron and ferritin is poor when iron stores are depleted 3