What it measures.
Serum iron measures the amount of iron circulating in your blood bound to transferrin protein. Iron is essential for hemoglobin production, oxygen transport, energy metabolism, and immune function. Unlike ferritin (storage iron), serum iron fluctuates throughout the day and with recent meals, making it less reliable alone but valuable as part of a complete iron panel.
The concentration of iron bound to transferrin in blood at the time of the test, measured in μg/dL or μmol/L. This represents iron in transit—being delivered to tissues for use or being recycled from old red blood cells.
Why it matters.
Iron is critical for oxygen delivery, energy production, and countless enzymatic reactions. Both deficiency and excess are harmful. Serum iron helps diagnose iron deficiency (most common nutritional deficiency) and iron overload (hemochromatosis), though it should be interpreted with ferritin and TIBC.
Physiology.
Iron from diet is absorbed in the duodenum, bound to transferrin for transport, stored as ferritin, and used primarily for hemoglobin synthesis. The body tightly regulates iron via hepcidin hormone. Serum iron represents iron actively being transported—only about 4-5 mg of the body's total 3-4 grams.
Testing & preparation.
How to prepare
- Fast for 8-12 hours (iron rises after meals)
- Morning testing preferred (iron highest in AM)
- Avoid iron supplements for 24-48 hours before testing
- Note: Serum iron alone is insufficient—need full iron panel
When to test
As part of complete iron panel (with ferritin, TIBC, transferrin saturation) when investigating anemia or fatigue
How often
Every 3-6 months when actively treating deficiency; annually for maintenance
Interpretation.
High serum iron
Common causes:
- Hemochromatosis (genetic iron overload)
- Recent iron supplement or infusion
- Liver disease (releasing stored iron)
- Hemolytic anemia (rapid RBC destruction)
- Multiple blood transfusions
- African iron overload
Implications:
- If persistent with high ferritin: investigate hemochromatosis
- Iron overload damages liver, heart, pancreas, joints
- Increases oxidative stress and infection risk
- May accelerate aging and chronic disease
Low serum iron
Common causes:
- Iron deficiency (inadequate intake, poor absorption, blood loss)
- Anemia of chronic disease (inflammation blocks iron release)
- Recent blood donation or menstruation
- GI conditions affecting absorption (celiac, IBD, low stomach acid)
- Pregnancy (increased demand)
Implications:
- If ferritin also low: true iron deficiency
- If ferritin normal/high: anemia of chronic disease (inflammation)
- May explain fatigue, weakness, poor exercise tolerance
- Need to identify cause (diet vs. absorption vs. loss)
Optimization.
Diet
- Heme iron (meat, poultry, fish) is best absorbed
- Vitamin C dramatically enhances non-heme iron absorption
- Separate iron-rich foods from coffee, tea, calcium, phytates
- Cook in cast iron (adds iron to food)
- If overload: avoid vitamin C with iron foods, limit red meat
Lifestyle
- For deficiency: identify and address blood loss
- Time supplements/iron-rich meals away from inhibitors
- For overload: regular blood donation removes excess iron
- Avoid alcohol (worsens iron overload liver damage)
Supplements
- Iron bisglycinate or iron sulfate for deficiency
- Take with vitamin C on empty stomach for best absorption
- Avoid calcium within 2 hours of iron supplements
- Slow-release or every-other-day dosing reduces GI side effects
FAQs.
Why do I need a full iron panel, not just serum iron?
Serum iron alone is unreliable—it fluctuates with meals, time of day, supplements, and inflammation. A full iron panel (serum iron + ferritin + TIBC + transferrin saturation) tells the complete story: ferritin shows storage, TIBC shows transport capacity, and transferrin saturation (iron÷TIBC) shows how loaded your carriers are. Together they distinguish true deficiency from inflammation-related changes.
My serum iron is low but ferritin is normal—what does that mean?
This pattern suggests anemia of chronic disease or inflammation rather than true iron deficiency. Inflammation triggers hepcidin, which locks iron in storage (keeping ferritin normal) while blocking its release into blood (lowering serum iron). Check CRP for inflammation. True iron deficiency shows low serum iron AND low ferritin.
Can I take iron supplements without testing?
Risky. Iron is one supplement you should NOT take without testing. About 1 in 200-300 people have hereditary hemochromatosis—iron supplementation can accelerate organ damage. Even in non-hemochromatosis, excess iron is pro-oxidant and harmful. Test first, supplement only if deficient, and retest to confirm you've repleted.