Iron Blood Test: Ferritin, TIBC, Iron Saturation & How to Interpret Your Results
Diagnosing iron deficiency, iron overload, and anemia of chronic disease from your iron panel
Iron deficiency affects 14% of US adults and 2 billion people globally, yet it's routinely missed on standard blood work. Ferritin below 30 ng/mL indicates depleted stores; below 15 confirms deficiency. Transferrin saturation below 20% suggests functional iron deficit even when ferritin is normal.
Iron is essential for oxygen transport (hemoglobin), energy production (mitochondrial function), thyroid hormone synthesis, and immune function. Iron deficiency is the world's most common nutritional deficiency, affecting an estimated 2 billion people — and it causes symptoms long before anemia shows up on a CBC. Fatigue, difficulty concentrating, restless legs, exercise intolerance, and hair loss can all occur with depleted iron stores (low ferritin) while hemoglobin remains normal.
A 2025 JAMA review reinforced that serum ferritin is the most specific routine test for diagnosing iron deficiency, but emphasized that the traditional cutoff of 12-15 ng/mL misses many deficient patients. Current evidence supports a ferritin threshold of 30 ng/mL for iron deficiency in otherwise healthy individuals — and 70+ ng/mL when inflammation is present, since ferritin is an acute-phase reactant that rises with infection and chronic disease.
Iron Panel Reference Ranges
| Test | Normal Range | Optimal | What It Measures |
|---|---|---|---|
| Serum Iron | 60 – 170 μg/dL | 80 – 120 μg/dL | Iron circulating in blood right now |
| Ferritin | Men: 24 – 336 ng/mL / Women: 11 – 307 ng/mL | 40 – 150 ng/mL | Iron storage protein (most useful single test) |
| TIBC (Total Iron Binding Capacity) | 250 – 370 μg/dL | — | Transferrin capacity — rises when stores are low |
| Transferrin Saturation | 20 – 50% | 25 – 45% | % of transferrin carrying iron (Iron ÷ TIBC × 100) |
| Transferrin | 200 – 360 mg/dL | — | Iron transport protein |
Key diagnostic thresholds:
- Ferritin <15 ng/mL: Confirmed iron deficiency
- Ferritin <30 ng/mL: Iron deficiency (current evidence-based cutoff per 2025 JAMA review)
- Ferritin 30-70 ng/mL with inflammation (elevated CRP): Possible functional iron deficiency
- Transferrin saturation <20%: Functional iron deficit
- Ferritin >300 ng/mL (men) or >200 ng/mL (women): Investigate for iron overload (hemochromatosis)
Iron Deficiency vs. Anemia of Chronic Disease
Not all anemias are iron deficiency, and distinguishing the cause determines treatment:
| Marker | Iron Deficiency | Anemia of Chronic Disease | Both (Mixed) |
|---|---|---|---|
| Ferritin | Low (<30) | Normal or high | Low-normal (30-100) |
| TIBC | High (>370) | Low (<250) | Normal-low |
| Transferrin Sat | Low (<20%) | Low (<20%) | Low |
| Serum Iron | Low | Low | Low |
| CRP | Normal | Elevated | Elevated |
In anemia of chronic disease, the body actively sequesters iron to starve pathogens (a defense mechanism called nutritional immunity). Ferritin may be normal or even elevated because it's an acute-phase reactant. This is why a ferritin of 50 ng/mL with an elevated CRP doesn't rule out functional iron deficiency.
Common Causes of Iron Deficiency
- Menstruation: The #1 cause in premenopausal women. Heavy periods (menorrhagia) deplete stores faster than diet can replace them.
- Diet: Vegetarian/vegan diets provide only non-heme iron, which is 2-10× less bioavailable than heme iron from animal sources. Pair iron-rich plant foods with vitamin C to improve absorption.
- GI absorption issues: Celiac disease, H. pylori infection, inflammatory bowel disease, gastric bypass, and chronic PPI (proton pump inhibitor) use all impair iron absorption.
- Pregnancy: Iron requirements roughly double during pregnancy (from ~18mg/day to ~27mg/day).
- Occult GI bleeding: In men and postmenopausal women, iron deficiency should prompt evaluation for GI blood loss (colonoscopy, upper endoscopy) to rule out colon polyps, ulcers, or malignancy.
- Intense exercise: Endurance athletes (especially runners) have increased iron losses through foot-strike hemolysis, GI losses, and sweat. Ferritin below 30-40 ng/mL impairs performance even without frank anemia.
Frequently Asked Questions
What is ferritin in a blood test?
Ferritin is a protein that stores iron inside cells. Your serum ferritin level reflects your body's total iron reserves — it's the most useful single test for diagnosing iron deficiency. Low ferritin (below 30 ng/mL) indicates depleted iron stores, often before anemia develops on a CBC. However, ferritin is also an acute-phase reactant — it rises with infection and inflammation, so a 'normal' ferritin in someone with chronic inflammation doesn't rule out iron deficiency. In that case, transferrin saturation below 20% is a more reliable indicator.
What are symptoms of iron deficiency?
The classic '5 weird signs of iron deficiency' beyond just fatigue: (1) Pica — craving ice, dirt, or non-food items; (2) Restless legs syndrome — irresistible urge to move legs, especially at night; (3) Spoon-shaped nails (koilonychia); (4) Pagophagia — specifically craving and chewing ice; (5) Difficulty concentrating and 'brain fog.' Other symptoms include fatigue, weakness, exercise intolerance, shortness of breath, pale skin, cold hands/feet, brittle nails, and hair loss. Symptoms often appear with low ferritin before hemoglobin drops.
What is low iron saturation?
Low iron saturation (transferrin saturation below 20%) means less than 20% of your blood's iron-carrying protein (transferrin) is actually loaded with iron. It indicates your tissues aren't receiving adequate iron delivery, even if your ferritin appears normal. Low saturation with low ferritin = classic iron deficiency. Low saturation with normal/high ferritin = anemia of chronic disease (iron is locked in storage, not available for use). It's calculated as serum iron divided by TIBC times 100.
How long does it take to raise iron levels?
With oral iron supplementation (typically 325mg ferrous sulfate, containing 65mg elemental iron): symptoms may improve in 1-2 weeks, hemoglobin rises 1-2 g/dL per month, and ferritin normalizes over 3-6 months. Continue supplementation for 3 months after ferritin reaches 50+ ng/mL to fully replenish stores. Take iron with vitamin C on an empty stomach for best absorption. Every-other-day dosing may be as effective as daily (better absorption per dose, fewer side effects). IV iron works faster — ferritin can normalize in 1-2 weeks.
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