Low ferritin means your body’s iron reserves are running low. Ferritin is the protein that stores iron for future use, and when levels drop, it signals that you’re using more iron than you’re taking in. The most common causes are blood loss, not absorbing enough iron from food, increased demand from pregnancy or growth, and chronic inflammation. Many people experience symptoms like fatigue, difficulty concentrating, and restless legs even before their iron drops low enough to cause anemia.
How Your Body Stores and Uses Iron
Iron exists in two pools inside your body: a storage pool and a functional pool. The storage pool is your reserve, held inside ferritin molecules mainly in your liver. The functional pool is iron actively at work, carrying oxygen in red blood cells and supporting muscle and brain function. When your ferritin level is measured, you’re getting a snapshot of how full that storage pool is.
Iron enters your body through the upper part of the small intestine. From there, a transport protein carries it to either storage or active use. Anything that disrupts this pipeline, whether it’s losing blood, eating too little iron, or having trouble absorbing it, will drain your storage pool first. That’s why ferritin drops before you ever become anemic. A ferritin below 30 ng/mL is widely considered a sign of depleted iron stores, and growing evidence suggests your body starts compensating by ramping up iron absorption when ferritin falls below about 50 ng/mL. The WHO defines iron deficiency as ferritin below 15 ng/mL, at which point iron is essentially gone from storage sites and most people are already anemic.
Blood Loss: The Most Common Cause
Losing blood means losing iron, since most of your body’s iron is bound up in red blood cells. Heavy menstrual periods are one of the leading causes of low ferritin in women of reproductive age, and the effect is amplified during adolescence when rapid growth is also pulling from iron stores. Among women aged 25 to 54, iron deficiency affects roughly 10 to 21% depending on the threshold used.
Gastrointestinal bleeding is another major driver, and it’s often invisible. Peptic ulcers, abnormal blood vessels in the intestine, inflammatory bowel disease, and even long-term use of blood thinners can cause slow, steady blood loss that depletes ferritin over months. Frequent blood donation is a well-documented cause too, since each donation removes a significant amount of iron that takes weeks to replenish.
Dietary Factors and Iron Type
Not all dietary iron is created equal. Heme iron, found in meat, poultry, and fish, has a bioavailability of 15 to 35%, meaning your body absorbs a substantial portion of what you eat. Non-heme iron, the type found in plants, grains, and supplements, has a bioavailability of just 1 to 20%. A study of Dutch blood donors found that higher heme iron intake was directly associated with higher ferritin levels, while higher non-heme iron intake was actually linked to lower ferritin. The likely explanation: people eating more plant-based iron also tend to eat more foods rich in compounds called phytates and polyphenols (found in legumes, grains, coffee, and tea) that block iron absorption.
This doesn’t mean plant-based diets inevitably cause low ferritin, but it does mean they require more attention to iron intake. Coffee, tea, calcium supplements, and dairy consumed around mealtimes can all reduce how much iron you absorb. Vitamin C does the opposite, helping convert plant iron into a form your gut can take in more easily.
Conditions That Block Absorption
Even if you eat plenty of iron, your body may not be absorbing it properly. Iron absorption depends on a healthy upper intestine and adequate stomach acid to convert iron into its absorbable form. Several conditions interfere with one or both of those requirements.
Celiac disease is a major culprit. The immune reaction to gluten damages the intestinal lining, and iron deficiency anemia occurs in roughly 46% of people with celiac disease. Crohn’s disease and other inflammatory bowel conditions damage or inflame the same stretch of intestine where iron is absorbed. H. pylori infection, the bacterium behind most stomach ulcers, can cause low ferritin both by reducing absorption and by causing low-grade bleeding.
Stomach surgeries, particularly gastric bypass, reduce either the surface area available for absorption or the amount of stomach acid produced. Autoimmune gastritis, which destroys the acid-producing cells in the stomach lining, has the same effect. Long-term use of acid-reducing medications (proton pump inhibitors or H2 blockers) can also raise the risk of iron deficiency through reduced stomach acid.
Pregnancy and Periods of High Demand
During pregnancy, blood volume expands by about 35%, and the growing fetus, placenta, and maternal tissues all need iron. In the second and third trimesters, iron demand triples to roughly 5 mg per day. Most pregnant women who don’t supplement cannot maintain adequate iron stores during this period. After delivery, iron invested in the fetus and placenta is lost to the mother, though some iron from the expanded blood volume returns to storage. Experts estimate that a woman needs ferritin levels of 70 to 80 ng/mL at conception to complete a healthy pregnancy without supplementation or developing deficiency.
Children during rapid growth phases, especially adolescence, face a similar supply-and-demand mismatch. Their bodies are building new tissue and expanding blood volume quickly, and dietary intake often doesn’t keep pace.
Exercise and Iron Depletion in Athletes
Endurance athletes, particularly long-distance runners, are prone to low ferritin through several overlapping mechanisms. Repetitive impact from running can physically damage red blood cells in the feet, a phenomenon called foot-strike hemolysis. Iron is also lost through sweat and, in some cases, exercise-induced gastrointestinal bleeding and blood in the urine. On top of that, intense exercise triggers inflammation, which stimulates production of a hormone called hepcidin that temporarily blocks iron absorption in the gut. The cumulative effect is a steady drain on iron stores that routine dietary intake may not cover.
How Inflammation Complicates the Picture
Ferritin is not just a storage protein. It’s also part of the body’s inflammatory response, which creates a diagnostic blind spot. When you have chronic inflammation from conditions like inflammatory bowel disease, heart failure, kidney disease, or autoimmune disorders, your ferritin level can appear normal or even elevated while your body is actually iron-starved.
Here’s what happens: inflammatory signals trigger the release of hepcidin, the same hormone that rises during intense exercise. Hepcidin blocks iron from leaving storage sites and prevents iron absorption in the gut. So iron gets trapped inside cells where it can’t be used, and ferritin levels rise as part of the inflammatory response, even though the iron available for making red blood cells and supporting other functions is dangerously low. This is called functional iron deficiency. The standard “normal” ferritin ranges were established in healthy people without inflammation and don’t apply when inflammation is present. In these situations, the WHO raises the iron deficiency threshold to below 70 ng/mL, and additional markers like transferrin saturation (which measures how much iron is actually circulating and available) become essential for getting an accurate picture.
Symptoms Before Anemia Develops
One of the most important things to understand about low ferritin is that it causes symptoms long before it shows up as anemia on a standard blood count. Iron deficiency without anemia has been linked to fatigue, weakness, reduced exercise tolerance, difficulty concentrating, poor work productivity, irritability, and restless legs syndrome. It has also been associated with fibromyalgia-like symptoms and persistent symptoms in people being treated for hypothyroidism. In pregnant women, iron deficiency without anemia is connected to poorer neurodevelopmental outcomes in their infants.
Because hemoglobin often stays in the normal range until iron stores are severely depleted, many people with genuinely low ferritin are told their blood work looks “fine.” If your ferritin is below 30 ng/mL and you’re experiencing unexplained fatigue or brain fog, that number is worth investigating further, regardless of what your hemoglobin says.