How do iron and ferritin fit into fatigue?
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Short answer
Ferritin measures stored iron and is the single most useful test when fatigue is unexplained. The NIH Office of Dietary Supplements states that a serum ferritin below 30 mcg/L suggests iron deficiency and below 10 mcg/L suggests iron deficiency anaemia. Haemoglobin can stay normal while stores are empty, so a normal complete blood count does not exclude iron deficiency.
Get urgent care if: low iron is accompanied by any of the following.
- Chest pain, breathlessness at rest, or fainting
- Black tarry stools or visible blood in the stool
- Vomiting blood or material that looks like coffee grounds
- Bleeding heavy enough to soak through a pad every hour
Iron deficiency is the most commonly missed cause of fatigue in menstruating women, and it is missed because the wrong test is used to look for it. A complete blood count measures the red cells you currently have. Ferritin measures the iron in storage. Stores empty first, so a person can be iron deficient for months or years with a completely normal haemoglobin, normal mean cell volume and a report that says nothing is wrong.
What the ferritin number means
The NIH Office of Dietary Supplements states that a serum ferritin concentration below 30 mcg/L suggests iron deficiency, and a value below 10 mcg/L suggests iron deficiency anaemia. Many laboratories print a lower reference limit of 10 to 15 mcg/L, which is why a ferritin of 14 mcg/L is routinely reported as normal in a woman who is exhausted. Whether treating a ferritin between 30 and 50 mcg/L improves symptoms in people without anaemia is genuinely uncertain, and the trials that exist are small and inconsistent.
When ferritin is misleading
Ferritin is an acute phase protein. The NIH fact sheet notes that inflammation can raise serum ferritin, so infection, autoimmune disease, obesity, liver disease and alcohol use can all push it up while iron stores are low. MedlinePlus lists the same causes for a raised result. This is why ferritin is interpreted alongside a CRP and a transferrin saturation. A ferritin of 90 mcg/L with a CRP of 12 mg/L and a transferrin saturation of 9 percent is an iron deficient patient, not a replete one.
Low iron is a symptom, not a diagnosis
The reason for the deficiency matters more than the number. In menstruating women the usual cause is menstrual blood loss, and heavy periods deserve their own assessment. In men and in postmenopausal women, iron deficiency is a gastrointestinal question until proven otherwise, which usually means coeliac testing and consideration of endoscopy. Other causes include bariatric surgery, plant-based diets without attention to iron, chronic use of acid suppressing drugs, and blood donation.
| Result | What it suggests | Next step |
|---|---|---|
| Ferritin below 10 mcg/L | Iron deficiency anaemia likely | Treat and investigate the cause |
| Ferritin below 30 mcg/L | Iron deficiency | Treat and investigate the cause |
| Ferritin 30 to 100 mcg/L with high CRP | Possible masked deficiency | Add transferrin saturation |
| Normal ferritin, low transferrin saturation | Inflammation with limited iron availability | Look for the inflammatory cause |
| High ferritin with high transferrin saturation | Possible iron overload | Consider haemochromatosis testing |
If you are tired and have heavy periods, ask specifically for ferritin, transferrin saturation and CRP rather than just a blood count. Do not start iron supplements without testing first: iron overload is real, and taking iron blindly can delay finding the cause of blood loss.
The clinical detail
Oral iron is best absorbed on an empty stomach with vitamin C and away from calcium, tea and proton pump inhibitors. Alternate day dosing produces better fractional absorption than daily dosing because hepcidin rises for around 24 hours after a dose. Expect a reticulocyte response within about a week and a haemoglobin response over four to eight weeks, and plan three to six months of treatment after the haemoglobin normalises in order to refill stores.
Draw iron studies before the morning dose, since recent supplementation transiently raises serum iron and transferrin saturation. Ferritin should be interpreted with CRP in any inflammatory state. Intravenous iron is reasonable where oral iron is not tolerated, absorption is impaired, or losses outpace replacement.
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Sources
- Iron: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements
- Ferritin Blood Test. MedlinePlus, National Library of Medicine
- Anemia. MedlinePlus, National Library of Medicine
- C-Reactive Protein (CRP) Test. MedlinePlus, National Library of Medicine
This library is educational and is not individual medical advice. Read our Editorial and Medical Review Policy.