A ferritin of 20 ng/mL comes back unflagged at many laboratories, particularly for women, because lower limits vary by lab and by sex. It is also a number that would make most hematologists ask a question. Both of those things are true at once, and the gap between them is the subject of this page.
Three questions, not one
Most of the confusion here comes from running three separate ideas together.
The first is the reference interval, which is built from a population and tells you where you sit against other people the lab measured. The second is whether your iron stores are low. The third is whether you are anemic, which is a question about your red blood cells, not about your stores.
They come apart in a specific order. Stores run down first. The blood count holds. Anemia, if it comes at all, comes last. So a ferritin can be inside the reference interval, your hemoglobin can be entirely normal, and your stores can still be low.
Where does 20 sit? The World Health Organization defines depleted stores in an apparently healthy adult as a ferritin below 15. The British Society of Gastroenterology treats below 15 as absent stores and below 30 as generally indicating low body iron, and a 2025 JAMA review puts it the same way: in an adult without inflammation, a ferritin below about 30 ng/mL is diagnostic of iron deficiency, as is a transferrin saturation under 20 percent. The American Gastroenterological Association uses 45, but that number belongs to the narrower question of working up someone who already has iron deficiency anemia.
So it is not that every guideline calls 20 deficient in every adult. It is that in an adult without inflammation, 20 sits below the threshold most of that guidance uses, which makes it a real finding rather than a shrug.
What low iron actually does, and what it does not
Iron deficiency can exist before anemia develops, and symptoms can occur during that stage. The blood count is therefore not a complete measure of iron status.
Two symptom links have real evidence behind them. In a randomized trial of menstruating women with unexplained fatigue, a ferritin under 50 and a normal hemoglobin, twelve weeks of oral iron cut fatigue by about half, against under a third on placebo. And restless legs is one condition where the guidance uses iron thresholds of its own: iron treatment is considered when ferritin is 75 ng/mL or below or transferrin saturation is under 20 percent, and intravenous iron is generally favored when ferritin is between 75 and 100 ng/mL. The restless legs guidance says plainly that those thresholds are specific to restless legs and differ from what applies to the general population.
The rest is weaker, and I would rather say so. Hair shedding is a common reason a ferritin gets checked, and it is worth checking, but a controlled study of women with diffuse hair loss did not find iron deficiency more often than in women without it. Objective exercise capacity is less convincing than the fatigue data. There is no general symptom target for ferritin that I would defend as a number, which is why I do not publish one.
When inflammation is in the picture
Ferritin is an acute phase reactant. It rises with inflammation, infection, obesity, liver disease and alcohol, so a ferritin that looks acceptable can be sitting on top of a real deficiency. A 60 in someone with active inflammation is not the same finding as a 60 in someone whose inflammation is quiet.
So ferritin has to be interpreted differently when inflammation is present, which is why the WHO pairs it with inflammatory markers. Those markers and the rest of the iron studies, serum iron, TIBC and transferrin saturation, are what give a ferritin its context. Transferrin saturation helps most when a falsely reassuring ferritin is suspected, though it falls with inflammation too, so I read the whole set together rather than applying a rule to any one of them.
Where did the iron go
This is the part that matters most, and the part a lab report never asks.
The body has no regulated route for excreting iron. Iron balance is controlled largely through absorption, while iron is lost through bleeding and normal cell shedding. So if iron is low, either it is not going in or it is going out. In a menstruating woman, periods are the usual explanation, and a cycle heavy enough to drive a ferritin to 20 is worth treating in its own right.
In a man or a postmenopausal woman, unexplained iron deficiency deserves a cause. If anemia is present, gastrointestinal evaluation becomes especially important: the American and British gastroenterology guidelines recommend celiac serology, because impaired absorption is a common and quiet cause found in 3 to 5 percent of iron deficiency anemia, and upper and lower endoscopy. Without anemia, the risk of a gastrointestinal cancer is low and the British guideline does not recommend invasive investigation on the ferritin alone. The decision there is individualized on age, symptoms, history, how persistent the deficiency is, and other risk factors, with a lower threshold to investigate in men, in postmenopausal women, in anyone with gut symptoms and in anyone with a family history.
What I would not do is start iron, watch the number come up, and never ask the question. The ferritin will rise either way. Sometimes the answer underneath is a benign ulcer. Sometimes it is a colon cancer found early enough to matter.
Black or bloody stool, a change in bowel habit, or unexplained weight loss beside a low ferritin is not a question for an essay. Be seen.
The history, which no test replaces
A blood count with indices sits beside the iron studies because it can provide additional clues; B12 or folate testing is added when the history, symptoms, or blood-count pattern makes either plausible. Celiac disease belongs in the differential for unexplained iron deficiency, and serologic screening is routinely recommended in iron-deficiency anemia. Then the history does most of the work.
Cycle. Medications: two years or more on a proton pump inhibitor or an H2 blocker was associated with new iron deficiency in a study of 77,046 cases, and anti-inflammatories cause gut blood loss. Alcohol. Diet. Endurance training, which raises hepcidin, the hormone that shuts iron absorption down. Blood donation, a common answer nobody mentions: 15 percent of donors in one large study had absent iron stores, most often frequent female donors.
On actually correcting it
Three practical points, and on two of them the evidence is genuinely mixed.
Schedule. Alternate-day dosing absorbs a larger share of each dose, 21.8 against 16.3 percent in two small absorption trials in iron-depleted women, because daily dosing raises hepcidin, which blocks the next dose. But a 2025 meta-analysis of 11 trials found hemoglobin recovery similar on either schedule, slightly favoring daily, with side effects much the same. The British guideline starts daily and moves to every other day for tolerance; the AGA advises no more than once daily and notes every-other-day may be better tolerated with similar absorption. I choose between them on the formulation, the severity, what the person tolerates and how they respond, rather than assuming one schedule wins.
Vitamin C. It increases iron absorption chemically, which is why it became standard advice, and a randomized trial of 440 adults with iron deficiency anemia found that adding 200 mg to oral iron made no meaningful difference to hemoglobin or ferritin. The 2024 AGA practice update still advises adding it. Given that, I do not treat it as essential for every patient. Keeping iron away from calcium, coffee and tea remains reasonable; those inhibit absorption in single-meal studies.
Follow-up. How soon depends on how deficient someone is and what they are taking. I recheck to confirm the expected response actually happened, and continue iron long enough to refill the stores rather than stopping when the blood count looks right. If the ferritin is not moving the way it should, the assumption about where the iron was going was wrong, and that is worth knowing.
The point
Ferritin answers a different question from hemoglobin. Iron stores can run down before anemia appears, and inflammation can make a ferritin look reassuring when it is not.
The work is not chasing one number. It is four questions: whether iron deficiency is present, why it developed, whether it explains how you feel, and whether the response to treatment confirms that it did.
If you have a ferritin in the teens or twenties and nobody has asked where the iron went, that is the kind of reading the Precision Health Assessment exists for.
