Biomarker comparison
Ferritin vs serum iron: what each measures, and which one shows iron deficiency
Short answer
Serum iron measures the iron travelling in the blood at the moment of the draw — a number that swings with the time of day, the last meal, a supplement taken yesterday and any inflammation. Ferritin measures the iron held in storage, and it is the first thing to fall when the body runs short, which makes it the test for iron deficiency: a ferritin under 30 ng/mL is deficiency even when the hemoglobin and the serum iron are still normal. The catch is that ferritin also rises with inflammation, so when a CRP is up or a chronic disease is present the iron saturation is read alongside it.
Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026
Ferritin vs Iron side by side
| Ferritin | Iron | |
|---|---|---|
| What it measures | The storage protein for iron; blood ferritin tracks total body stores | Iron bound to transferrin in the blood — the amount in transit at that moment |
| Normal range (adults) | Male, age 18+: 24 – 336 ng/mL; Female, age 18+: 11 – 307 ng/mL | Male, age 18+: 65 – 175 mcg/dL; Female, age 18+: 50 – 170 mcg/dL |
| What it tells you | How much iron is in reserve | How much iron is being delivered right now; with TIBC it gives the saturation |
| Varies with | Inflammation, infection, liver disease, alcohol, obesity (all raise it) | Time of day (highest in the morning), meals, supplements in the last 24 hours, inflammation (lowers it) |
| A low value means | Iron deficiency — the earliest and most specific sign | Deficiency or inflammation; cannot tell them apart on its own |
| A high value means | Inflammation or fatty liver far more often than iron overload; overload when saturation is also above 45% | A recent supplement or meal, hemolysis or liver injury, or iron overload (hemochromatosis) when persistent |
| Best use | Diagnosing deficiency and tracking replacement | Screening for overload (with saturation) and interpreting a ferritin that inflammation may have raised |
Reference ranges are the reviewed catalog ranges for adults; your laboratory's printed range may differ slightly.
Why ferritin is the test for deficiency
Iron is lost in stages. First the stores empty — ferritin falls while hemoglobin and serum iron remain normal; then transport iron drops and saturation falls; only last does hemoglobin fall and anemia appear. A low ferritin therefore catches deficiency months before anemia, and it is specific: outside of a few rare conditions, nothing but low iron stores makes ferritin low. The British Society of Gastroenterology and WHO both use a ferritin under 15–30 ng/mL to define deficiency, with 30 the practical cut-off in adults and 45 when inflammation may be present.
Serum iron cannot do that job. A normal-weight adult's serum iron can vary by 30% between morning and afternoon, doubles after an iron tablet, and falls with any infection or inflammation. A single serum iron in the normal range says nothing about the stores behind it.
When serum iron is the useful one
Serum iron earns its place as part of the 'iron studies' set — with total iron-binding capacity (TIBC) or transferrin it gives the transferrin saturation (iron divided by TIBC). Saturation is the screening test for iron overload: a fasting saturation above 45% is the trigger for an HFE gene test for hemochromatosis, whatever the ferritin. It is also the tiebreaker when ferritin is uninterpretable because of inflammation — a saturation under 20% with a ferritin under 100 still means deficiency in someone with a chronic disease.
The inflammation trap
Ferritin is an acute-phase protein: infection, inflammatory disease, cancer, liver disease, obesity and heavy alcohol use all push it up, sometimes into the hundreds, while the stores behind it may be empty. A ferritin of 80 in someone with rheumatoid arthritis, inflammatory bowel disease or heart failure can hide a real deficiency. The fix is to read it with a CRP and the saturation, and in chronic disease to use the higher cut-off (under 100 with saturation under 20%).
Reading the four together
The common patterns:
- Iron deficiency — low ferritin, low or low-normal serum iron, high TIBC, saturation under 20%.
- Anemia of inflammation — normal or high ferritin, low serum iron, low TIBC, saturation low-normal; the body is hiding iron from microbes.
- Iron overload (hemochromatosis) — high ferritin, high serum iron, saturation above 45%; confirmed by the HFE gene test.
- Recent supplement or meal — high serum iron and saturation with an unchanged ferritin; repeat fasting and off iron for 24 hours.
How do you test Ferritin and Iron?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Baseline Anemia Panel | Panel · 5 tests | $74.99 | Yes | 2–4 business days |
| Women's Health Panel | Panel · 8 tests | $129.99 | Yes | 2–5 business days |
| Fitness & Performance Panel | Panel · 8 tests | $119.99 | Yes | 2–5 business days |
| Iron Panel (Iron, TIBC & Ferritin) | Test · 4 biomarkers | $32.99 | Yes | 2–5 business days |
Common questions
Can you have normal iron but low ferritin?
Yes — it is the typical early picture. The stores (ferritin) empty first while the iron in transit (serum iron) is still being maintained, and hemoglobin falls last. A low ferritin with a normal serum iron and hemoglobin is iron deficiency without anemia, and it causes fatigue, hair shedding and restless legs in its own right.
Which is more accurate, ferritin or iron?
For finding or excluding iron deficiency, ferritin — it reflects the stores and is stable from hour to hour. Serum iron is accurate for what it measures (iron in transit at that moment) but that number changes with the time of day, meals, supplements and inflammation, so it cannot diagnose deficiency on its own. For overload, the serum-iron-derived saturation is the screening test.
Why is my ferritin high but iron low?
That combination is the signature of inflammation or chronic disease: the body raises ferritin and locks iron away from circulation in response to infection, inflammatory disease, cancer, liver disease or obesity. It can coexist with true deficiency; a saturation under 20% and a ferritin under 100 in that setting still means the stores need filling.
Should I fast for an iron and ferritin test?
Ferritin does not need fasting. Serum iron and saturation are best drawn in the morning after an overnight fast, with no iron supplement for 24 hours and no vitamin C or multivitamin that morning — a tablet the night before can double the serum iron and mislead.
Sources
- Snook et al., British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults (Gut 2021)
- World Health Organization, Guideline on use of ferritin concentrations to assess iron status in individuals and populations (2020)
- NIH Office of Dietary Supplements — Iron: Fact Sheet for Health Professionals
- Mayo Clinic Labs
- British Society of Gastroenterology — Guidelines for the Management of Iron Deficiency Anaemia in Adults (2021)
- MedlinePlus (NIH) — Iron Tests
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.