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Extreme fatigue and difficulty concentrating can be due to hidden, depleted iron stores, even though the usual blood value looks completely normal. Measuring ferritin reveals this hidden iron deficiency, which is most often caused by heavy menstruation, a one-sided diet or poor absorption in the intestine. While low values in men and women after menopause must always be investigated clinically to rule out hidden bleeding, the deficiency in others is best treated with iron supplements, which are then followed up carefully to ensure that the stores are actually recovering.
Ferritin may be the missing test when someone is seeking help for fatigue, lack of energy or poor recovery but is told that "the blood count looks normal". Hemoglobin can be within the reference range while the body's iron stores are already depleted. In this case, a low ferritin can provide an explanation for symptoms that are otherwise easily dismissed as stress, poor sleep or "just a lot right now".
What is ferritin and why is a normal blood count not enough?
Ferritin is a protein that stores iron in the body. A ferritin value in the blood therefore primarily reflects how large your iron stores are, while hemoglobin shows how much oxygen-carrying pigment is in the red blood cells.
This means that iron deficiency can develop in stages. First, the stores are depleted, which is often seen as low ferritin. Only later does hemoglobin drop so much that it is classified as anemia, or anemia. Therefore, standard tests such as blood counts can be normal even though the body already has too little available iron.
For many, this is an aha experience. They have been tested for fatigue, headaches, poorer fitness or difficulty concentrating, but have been told that "no errors are visible". If ferritin has not been analyzed, iron deficiency without anemia may have been missed.
Low ferritin can cause symptoms even without anemia
Iron is not only needed to form hemoglobin. It is also important for muscle function, energy metabolism and several enzyme systems in the body. Therefore, iron deficiency can cause problems even before the blood value becomes low.
Common symptoms of low ferritin are:
pronounced fatigue or unusual exhaustion
reduced physical energy and poorer exercise capacity
dizziness, palpitations or shortness of breath during exertion
difficulty concentrating and “brain fog”
restless legs
headache, chills or general weakness
Research on adults with iron deficiency without anemia shows that iron therapy can reduce fatigue in certain groups, especially menstruating women with low or borderline low ferritin levels. Iron therapy has been shown to relieve fatigue and exhaustion even when the hemoglobin value is completely normal. There is clear support that iron replacement can improve energy in adults with non-anemic iron deficiency, although the actual effect varies from person to person.
At the same time, it is important to interpret symptoms in a nuanced way. Fatigue is common and can be due to lack of sleep, stress, thyroid disease, depression, infection, B12 or folate deficiency and many other conditions. A low ferritin is therefore a possible explanation, not the only one.
How is ferritin interpreted in practice?
Ferritin is the most useful single marker for assessing iron deficiency. In otherwise healthy adults, low ferritin is defined as below 15 µg/L, but in everyday clinical practice, the limit below 30 µg/L is often used as a clear indication of iron deficiency. In the case of symptoms and concomitant supporting disease, even higher levels may be relevant, especially if inflammation is present.
Here a common misconception arises: a “normal” ferritin is not always normal in a medical sense. Ferritin is also an acute phase protein, which means that the value can rise in the event of infection, inflammation, liver disease, obesity or other concomitant morbidity. In this case, a ferritin that looks acceptable can hide actual iron deficiency.
When inflammation is suspected, it is therefore often more informative to interpret ferritin together with other tests, such as transferrin saturation, CRP and blood status. Several reviews and guidelines state that ferritin up to around 100 µg/L can still be compatible with iron deficiency in chronic inflammation, and that transferrin saturation below 20 percent strengthens the diagnosis.
It is also good to know that serum iron itself is a poorer test for making the diagnosis. Iron levels in the blood vary throughout the day and are affected by both diet and temporary factors, while ferritin better reflects the body's stores.
Why does the ferritin value drop?
For fertile women, heavy menstruation is the most common explanation, as regular blood loss gradually depletes the stores without the body having time to recover. Another common cause is poor absorption in the intestine, which can be due to celiac disease, inflammatory bowel disease, gastric surgery or long-term use of antacids. Even a one-sided diet or vegetarian and vegan diet without a well-thought-out iron plan plays a big role, just as regular blood donation and hard exercise can deplete reserves without you noticing.
When low ferritin is detected in men or in women after menopause, a medical investigation is always required, since heavy menstruation is no longer an explanation and hidden bleeding in the gastrointestinal tract or other underlying diseases must then be ruled out.
When is it time to test ferritin and what happens next?
It is especially wise to measure ferritin if you experience persistent fatigue despite a normal blood value, if you have heavy menstruation, exercise a lot or want to follow up on previous iron treatment. If the test shows low values, the next step is to both treat the deficiency and investigate the underlying cause. Iron tablets are the most common treatment, but because they often cause stomach upset and constipation, careful monitoring is required to ensure that the levels actually rise and that the symptoms disappear.
