Anemia develops when your blood lacks enough healthy red blood cells or hemoglobin to carry oxygen efficiently, and the underlying reason varies enormously from person to person. Iron deficiency is the single most common cause worldwide, but a shortage of vitamin B12, chronic inflammation, kidney disease, genetic conditions, and even certain medications can all drive your red blood cell count down. Figuring out which cause applies to you matters because the treatment for one type of anemia can be useless or even harmful for another.
How Doctors Start Narrowing It Down
One of the first things a blood test reveals is the size of your red blood cells, measured as the mean corpuscular volume (MCV). Doctors use that number to sort anemia into three broad categories: microcytic (small cells), normocytic (normal-sized cells), and macrocytic (large cells).1Mayo Clinic Proceedings. Anemia in Adults: A Contemporary Approach to Diagnosis Small red blood cells point strongly toward iron deficiency or inherited hemoglobin disorders. Large red blood cells suggest a B12 or folate problem. Normal-sized cells can accompany chronic disease, kidney failure, or bone marrow problems. This single lab value doesn’t give you the final answer, but it cuts the list of suspects roughly in thirds.2Family Practice. The accuracy of mean corpuscular volume guided anaemia classification in primary care
Iron Deficiency
Iron is a building block of hemoglobin, and when your body runs low, it produces smaller, paler red blood cells that carry less oxygen. The result is the classic symptoms: fatigue, lightheadedness, pale skin, and sometimes a craving for ice or other non-food items. But how your body manages iron is more tightly controlled than most people realize. A hormone called hepcidin, made in the liver, acts as the master switch. Hepcidin controls a protein called ferroportin, the only known exporter of iron from your intestinal cells into your bloodstream.3PubMed Central. The role of hepcidin, ferroportin, HCP1, and DMT1 protein in iron absorption in the human digestive tract When hepcidin levels rise, ferroportin gets broken down, and less iron makes it from your gut into circulation. When hepcidin falls, the gates open and iron absorption increases.4PubMed Central. Hepcidin and Iron in Health and Disease
This matters because not every iron problem is about eating too little iron. Your diet could be adequate, but if hepcidin is abnormally elevated from chronic inflammation, the iron sits trapped in your gut lining and never reaches your blood. Ferroportin also moves iron out of macrophages, the immune cells that recycle iron from old red blood cells. So when hepcidin is high, recycled iron gets locked away too.5Biochimica et Biophysica Acta (BBA) – Molecular Cell Research. Hepcidin and iron homeostasis
Heavy Menstrual Bleeding
For women of reproductive age, the leading cause of iron deficiency isn’t diet. It’s blood loss through menstruation. Heavy menstrual bleeding is extremely common and is the top driver of iron deficiency and iron deficiency anemia in this population.6PubMed. Heavy menstrual bleeding, iron deficiency, and iron deficiency anemia: Framing the issue Yet the link gets overlooked surprisingly often. Many women grow up accepting heavy periods as normal and never connect their fatigue or brain fog to iron loss. Others have their iron levels checked but never get asked about their menstrual pattern.
A woman losing a large volume of blood every month can outpace what even a good diet replaces, because the body absorbs only a small fraction of dietary iron at any given time. Oral iron supplements help, but they’re poorly tolerated by many people due to constipation and nausea, which leads to inconsistent use. Addressing the bleeding itself, whether through hormonal therapy or other medical treatment, is often necessary to break the cycle.7PubMed. The relationship between heavy menstrual bleeding, iron deficiency, and iron deficiency anemia
Vitamin B12 Deficiency
B12 is essential for making DNA in rapidly dividing cells, and red blood cell precursors divide constantly. When B12 is scarce, those precursors grow abnormally large and can’t mature properly, producing the oversized red blood cells doctors see on a blood smear. The anemia that results is called megaloblastic anemia.
The most well-known cause of severe B12 deficiency is pernicious anemia, an autoimmune condition in which the body attacks the stomach cells that produce intrinsic factor, a protein you need to absorb B12 from food. Pernicious anemia is diagnosed based on the presence of immune gastritis along with antibodies against intrinsic factor or parietal cells.8PubMed. Oral vitamin B12 supplementation in pernicious anemia: a prospective cohort study Without intrinsic factor, even a diet rich in B12 won’t prevent deficiency. The traditional treatment has been B12 injections, though research into high-dose oral supplementation is ongoing.
You don’t need pernicious anemia to become B12 deficient, though. Strict vegans and vegetarians are at risk because B12 occurs naturally almost exclusively in animal products. Older adults often lose stomach acid production gradually, which impairs the release of B12 from food proteins. And as we’ll see, certain medications can push you toward deficiency as well.
Medications That Interfere with B12 Absorption
Two of the most widely prescribed drug classes in the world quietly chip away at B12 levels. Proton pump inhibitors (PPIs), the acid-reducing drugs taken for heartburn and reflux, and metformin, the first-line treatment for type 2 diabetes, both reduce the body’s ability to absorb B12.9Advances in Nutrition. Proton Pump Inhibitors, H2-Receptor Antagonists, Metformin, and Vitamin B-12 Deficiency: Clinical Implications PPIs do this by suppressing stomach acid, which is needed to free B12 from food proteins. Metformin appears to interfere with B12 uptake in the small intestine.
Individually, either drug raises the risk modestly. But a large matched-cohort study found that taking metformin and a PPI together raised the risk of B12 deficiency significantly compared to metformin alone.10PubMed Central. Concomitant use of metformin and proton pump inhibitors increases vitamin B12 deficiency risk in type 2 diabetes Given that many people with type 2 diabetes also take a PPI for reflux, this overlap is far from rare. If you’ve been on both medications for years and feel increasingly tired, a B12 check is worth requesting.
Anemia of Chronic Disease
This is the type of anemia that confuses people most, because it doesn’t come from a nutritional deficiency at all. In conditions like rheumatoid arthritis, inflammatory bowel disease, chronic infections, and many cancers, the body’s inflammatory signals ramp up hepcidin production. That elevated hepcidin traps iron inside macrophages and blocks intestinal absorption, starving the bone marrow of iron even when the body’s overall iron stores are normal or even high.11European Journal of Internal Medicine. Pathophysiology, diagnosis and treatment of anemia of chronic disease
The practical consequence: a standard ferritin test can look perfectly fine, because there’s plenty of iron in storage. But that iron isn’t being released where it’s needed. This makes anemia of chronic disease easy to miss or misinterpret. Taking iron supplements in this situation often does little good and can sometimes make things worse, since the iron has nowhere productive to go. Treatment focuses on managing the underlying inflammatory condition rather than piling on more iron.
Kidney Disease
Your kidneys do more than filter waste. Specialized cells in the kidneys produce erythropoietin (EPO), the hormone that signals your bone marrow to make red blood cells. As kidney function declines, EPO production drops, and anemia develops as a direct result.12PubMed. Physiology and pathophysiology of renal erythropoietin-producing cells In chronic kidney disease, EPO levels are inappropriately low relative to the degree of anemia. The deficiency starts early in the disease course, but it becomes more severe as kidney function falls below a certain threshold.13Frontiers in Medicine. Anemia in Chronic Kidney Disease: From Pathophysiology and Current Treatments, to Future Agents
Kidney-related anemia is one reason people with advanced kidney disease often feel exhausted. Treatment typically involves synthetic EPO injections, though iron supplementation is usually needed alongside them because EPO drives the marrow to ramp up production, which quickly depletes whatever iron is available.
When Red Blood Cells Are Destroyed Too Fast
Sometimes the problem isn’t that your body makes too few red blood cells, it’s that red blood cells are being destroyed faster than the marrow can replace them. This category, called hemolytic anemia, has several distinct triggers.
In autoimmune hemolytic anemia, the immune system produces antibodies that tag red blood cells for destruction. Some forms of this disease depend heavily on the complement system, a cascade of immune proteins that punch holes in cells. Cold agglutinin disease, for example, is entirely complement-driven, while warm-antibody hemolytic anemia involves complement in a more limited way.14PubMed Central. Red blood cell destruction in autoimmune hemolytic anemia: role of complement and potential new targets for therapy
G6PD deficiency is a different mechanism entirely. G6PD is an enzyme that protects red blood cells from oxidative damage. People who inherit a deficiency in this enzyme have red blood cells that are vulnerable to breaking apart when exposed to certain triggers, including specific medications, infections, or fava beans.15PubMed. Glucose-6-phosphate dehydrogenase deficiency The enzyme is critical for generating the molecules that neutralize harmful oxygen radicals inside the cell. Without enough of it, a triggering event can cause sudden, dramatic hemolysis.16PLOS ONE. Markers of oxidative stress in umbilical cord blood from G6PD deficient African newborns G6PD deficiency is the most common red blood cell enzyme disorder worldwide and is especially prevalent in populations from malaria-endemic regions.17PubMed Central. Acute Hemolytic Anemia Due to Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency Triggered by Helicobacter pylori Quadruple Therapy in a Jehovah’s Witness: A Case Report
Inherited Hemoglobin Disorders
Sickle cell disease and beta-thalassemia are both caused by mutations in the gene for beta-globin, the protein that forms half of the hemoglobin molecule.18PubMed. Genetic Basis and Genetic Modifiers of β-Thalassemia and Sickle Cell Disease In sickle cell disease, the mutation produces hemoglobin that distorts red blood cells into a rigid crescent shape under low-oxygen conditions. Those sickled cells get stuck in small blood vessels, causing pain crises and organ damage, and they break down much faster than normal cells. In beta-thalassemia, the mutation reduces or eliminates production of the beta-globin chain itself, leading to mismatched hemoglobin components and ineffective red blood cell production.
Both conditions are inherited in a recessive pattern. Carrying one copy of a sickle cell mutation (sickle cell trait) usually causes no anemia. But inheriting two copies, one from each parent, results in full-blown disease. These conditions are lifelong and managed with a combination of transfusions, medication, and in some cases newer gene-based therapies.
Bone Marrow Failure
Aplastic anemia is one of the more serious causes on this list. In acquired aplastic anemia, the immune system attacks the stem cells in the bone marrow that give rise to all blood cell lines. The result is not just anemia but also low white blood cells and low platelets, a combination called pancytopenia. The bone marrow becomes hypocellular, meaning mostly empty fat where blood-forming cells should be. Treatment ranges from immunosuppressive drugs to bone marrow transplantation, depending on severity and the patient’s age.
Myelodysplastic syndromes, a group of bone marrow disorders more common in older adults, also cause anemia through defective red blood cell production. In these conditions, the marrow produces cells that are abnormal in shape and die before they mature. Cancers that invade the marrow, such as leukemia or metastatic solid tumors, can crowd out normal blood cell production too.
Pregnancy
Mild anemia during pregnancy is so common that many women assume it’s unavoidable. There’s a physiological basis for this: during pregnancy, plasma volume expands substantially to support the growing fetus, and this expansion is considered an important marker of a healthy pregnancy.19PubMed Central. Emerging understanding and measurement of plasma volume expansion in pregnancy Because the liquid portion of blood increases faster than the red blood cell count, hemoglobin concentration drops even if red blood cell production is actually increasing. This dilutional effect is normal to a point.
On top of that, the fetus draws heavily on the mother’s iron and folate stores, especially in the third trimester. A woman who enters pregnancy with borderline iron stores can quickly tip into true iron deficiency. Prenatal vitamins with iron and folate exist partly for this reason, and screening for anemia during pregnancy is standard practice.
Less Obvious Culprits
A few causes of anemia tend to fly under the radar because they aren’t the first thing doctors think of.
Thyroid disease is one. Hypothyroidism, an underactive thyroid, can contribute to anemia through several pathways at once: reduced bone marrow activity, lower erythropoietin production, and increased susceptibility to iron, B12, or folate deficiency.20PubMed. Anemia in thyroid diseases The anemia often resolves once the thyroid condition is treated, but it can persist if a co-existing nutritional deficiency isn’t also addressed.
Heavy alcohol use causes anemia through multiple routes. Alcohol directly damages developing red blood cells in the bone marrow, leading to enlarged cells (macrocytosis) even in the absence of any vitamin deficiency. On top of that direct toxic effect, chronic drinkers are often folate-depleted because of poor dietary intake and impaired absorption, compounding the problem.21PubMed. Folate deficiency in the alcoholic–its relationship to clinical and haematological abnormalities, liver disease and folate stores
Zinc supplements, often taken by older adults for immune support, can quietly cause copper deficiency if taken in excess. Copper is needed for proper iron metabolism, and its depletion from zinc over-supplementation can lead to anemia along with neurological symptoms like tingling and unsteadiness.22PubMed Central. Zinc-Induced Copper Deficiency as a Rare Cause of Neurological Deficit and Anemia
Lead exposure remains a relevant cause in certain settings, particularly in older housing with lead paint and in some occupational environments. Lead disrupts heme biosynthesis, the chemical pathway that produces the oxygen-carrying part of hemoglobin, resulting in a mild sideroblastic anemia.23PubMed. Lead toxicity and heme biosynthesis Children are especially vulnerable because they absorb lead more efficiently than adults do.
The Tricky Overlap Between Iron Deficiency and Chronic Disease
One of the most frustrating diagnostic puzzles in anemia care is telling true iron deficiency apart from anemia of chronic disease, because both can coexist in the same patient. Someone with rheumatoid arthritis, for example, might have inflammation-driven iron trapping and also genuine iron deficiency from poor appetite and NSAID-related gut bleeding. Standard blood markers like ferritin become unreliable in this scenario because inflammation artificially inflates ferritin levels, making iron stores look adequate when they aren’t.
A lab marker called soluble transferrin receptor (sTfR) helps here. Unlike ferritin, sTfR rises when the body is truly iron-deficient and isn’t pushed around by inflammation. A ratio combining sTfR with ferritin (the sTfR/log ferritin index) performs even better at distinguishing the two conditions and detecting hidden iron deficiency layered on top of chronic disease.24PubMed. Improved differential diagnosis of anemia of chronic disease and iron deficiency anemia: a prospective multicenter evaluation of soluble transferrin receptor and the sTfR/log ferritin index The index has been shown to outperform sTfR alone, and neither marker is significantly affected by the inflammatory state itself.25PubMed. Soluble transferrin receptor and transferrin receptor-ferritin index in iron deficiency anemia and anemia in rheumatoid arthritis If you have a chronic inflammatory condition and your iron panel results seem contradictory, asking about sTfR testing can be genuinely useful.
Gut Problems and Malabsorption
You could eat an iron-rich or B12-rich diet and still become deficient if your gut can’t absorb nutrients properly. Celiac disease is one of the more common culprits. The damage to the small intestinal lining in undiagnosed celiac disease impairs iron, folate, and sometimes B12 absorption. People occasionally get diagnosed with celiac disease only because persistent anemia prompted a workup that eventually led to the right test.
Inflammatory bowel disease, particularly Crohn’s disease affecting the terminal ileum where B12 is absorbed, can produce chronic deficiency. Surgical removal of portions of the stomach or small intestine, including weight-loss surgeries like gastric bypass, also reduces the absorptive surface area for iron and B12. These patients typically require lifelong monitoring and supplementation.
Even conditions that seem unrelated to the gut, like long-term antibiotic use disrupting the microbiome or parasitic infections damaging the intestinal wall, can impair nutrient absorption enough to cause anemia over time. If your anemia keeps coming back despite supplementation, a gut-related cause deserves investigation.