Iron deficiency anemia is a condition where your body doesn’t have enough iron to produce adequate healthy red blood cells. It’s the most common type of anemia worldwide, affecting roughly 40% of young children, 37% of pregnant women, and 30% of women of reproductive age according to the World Health Organization. Without enough iron, your red blood cells become smaller and carry less oxygen, which is why the hallmark symptoms are fatigue, weakness, and shortness of breath.
How Iron Deficiency Develops in Stages
Iron deficiency doesn’t happen overnight. It progresses through distinct stages, and you can be iron-deficient for a long time before it shows up as full-blown anemia.
In the first stage, your body is using more iron than it’s taking in. Your bone marrow gradually burns through its stored reserves, and your gut compensates by absorbing more iron from food. At this point, blood tests may still look normal, but your iron stores are quietly dropping. In the second stage, those stores have fallen low enough that your body can no longer produce red blood cells at its usual rate. Your transferrin saturation, a measure of how much iron is actually available for use, drops below 20%. In the third and final stage, red blood cells start coming out noticeably smaller and paler than normal. This is the stage most people associate with iron deficiency anemia: the point where you feel it.
The reason iron matters so much is that it’s a core component of hemoglobin, the protein inside red blood cells that carries oxygen from your lungs to every tissue in your body. Iron gets inserted into a molecule called protoporphyrin inside your bone marrow to form heme, which becomes part of hemoglobin. Without enough iron, this assembly process stalls, and you end up with red blood cells that are undersized and inefficient at delivering oxygen.
Common Causes
There are really only three ways to become iron deficient: you’re not absorbing enough, you’re not eating enough, or you’re losing too much.
Blood loss is the most common cause in adults. For premenopausal women, heavy menstrual periods are the leading driver. In men and postmenopausal women, the culprit is often slow, chronic bleeding from the gastrointestinal tract, sometimes from ulcers, polyps, or colorectal cancer. This is why new iron deficiency anemia in an older adult often prompts a workup to rule out internal bleeding.
Several digestive conditions impair iron absorption even when your diet contains plenty of it. Celiac disease damages the lining of the small intestine where iron is absorbed. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis do the same. Peptic ulcers, chronic kidney disease, and surgeries that bypass or remove part of the small intestine (including bariatric surgery) can all reduce your ability to absorb iron from food.
Dietary insufficiency alone can also cause it, particularly in vegetarians and vegans who rely exclusively on plant-based iron sources, which are harder for the body to absorb. Pregnancy is another major risk factor because the body’s iron demands roughly double to support the growing baby and expanded blood volume.
Symptoms to Recognize
The early signs of iron deficiency anemia are easy to dismiss. Fatigue and low energy come first, often chalked up to poor sleep or stress. As anemia worsens, you may notice pale skin, dizziness, cold hands and feet, headaches, and a faster-than-normal heartbeat, especially during physical activity. Shortness of breath doing things that used to feel easy, like climbing stairs, is a classic red flag.
Some symptoms are more distinctive. Pica, a craving for non-food items like ice, dirt, or starch, is surprisingly common in iron deficiency and often resolves once iron levels are restored. A smooth, sore tongue (from loss of the normal tiny bumps on its surface), cracks at the corners of your mouth, and brittle or spoon-shaped nails that curve inward are all physical signs that point specifically toward iron deficiency rather than other causes of anemia.
How It’s Diagnosed
A standard complete blood count is usually the first test. In iron deficiency anemia, the mean corpuscular volume (a measure of red blood cell size) drops below 80 femtoliters, indicating smaller-than-normal cells. But the key confirmatory test is serum ferritin, which reflects your body’s iron stores.
The World Health Organization has traditionally defined iron deficiency as a ferritin level of 15 ng/mL or lower. However, the American Gastroenterological Association has recommended raising that threshold to 45 ng/mL or lower, recognizing that many people with ferritin levels between 15 and 45 are genuinely iron-deficient and symptomatic. This is especially relevant if you have an inflammatory condition, because inflammation artificially raises ferritin levels and can mask a true deficiency.
Transferrin saturation below 20% is another reliable marker. Your doctor may also check total iron-binding capacity, which tends to rise when iron stores are low, as the body produces more of the transport protein transferrin in an attempt to scavenge whatever iron is available.
Dietary Iron: Heme vs. Non-Heme
Iron from food comes in two forms. Heme iron, found in meat, poultry, and fish, is absorbed significantly more efficiently than non-heme iron, which comes from plant sources like beans, lentils, spinach, and fortified grains. Most people get the majority of their dietary iron from non-heme sources, which makes absorption strategy important.
Vitamin C dramatically improves non-heme iron absorption. Eating citrus fruit, bell peppers, or tomatoes alongside iron-rich plant foods can make a meaningful difference. On the other hand, several common substances block non-heme iron absorption: tannins in tea and coffee, phytates in whole grains and legumes, bran fiber, and large amounts of calcium (particularly from supplements). Timing matters. Drinking coffee with a meal containing your main iron source reduces what you actually absorb.
Treatment With Oral Iron
For most people with iron deficiency anemia, treatment starts with oral iron supplements. Therapeutic doses typically range from 100 to 200 mg of elemental iron per day, depending on how severe the deficiency is and how well you tolerate the supplement.
The side effects are the main challenge. Nausea, constipation, dark stools, and stomach cramps are common enough that many people struggle to stick with daily dosing. If that’s the case, taking a lower dose or switching to every-other-day dosing can help. Every-other-day dosing appears to maintain good absorption while cutting down on gastrointestinal symptoms, though it takes longer to refill your iron stores. Different formulations (liquid, tablet, slow-release) may also make a difference in tolerability.
Replenishing iron stores is not fast. Symptoms like fatigue often start improving within a few weeks, but fully restoring your ferritin levels typically takes three to six months of consistent supplementation. Stopping too early is one of the most common reasons people end up iron-deficient again.
When Intravenous Iron Is Needed
Oral iron doesn’t work for everyone. If you have a condition that impairs absorption, like inflammatory bowel disease, celiac disease, or a history of bariatric surgery, iron taken by mouth may never reach your bloodstream in adequate amounts. Intravenous iron is recognized as appropriate first-line therapy in these situations, as well as for chronic kidney disease and chemotherapy-related anemia.
IV iron is also used when a faster response is needed. If you’re preparing for surgery and have less than six weeks to correct your anemia, or if your hemoglobin is severely low, IV iron can restore levels much more quickly than oral supplements. For moderate anemia (hemoglobin between 8 and 10.9 g/dL), a total dose of 1,200 to 1,500 mg is typical, with reassessment three months later. Severe cases with hemoglobin below 8 g/dL may require 2,000 to 3,000 mg total.
Blood transfusions are reserved for situations involving cardiovascular instability or debilitating symptoms. Professional guidelines specifically advise against transfusing red blood cells for iron deficiency when the patient is hemodynamically stable, since iron replacement achieves the same result without the risks of transfusion.
Long-Term Risks of Untreated Anemia
When iron deficiency anemia goes uncorrected for months or years, the consequences extend beyond fatigue. Your heart has to pump harder and faster to compensate for reduced oxygen delivery, and over time this extra workload can lead to an enlarged heart or heart failure. Cognitive function suffers too, particularly in children, where iron deficiency during critical developmental windows can impair learning, memory, and attention in ways that may not fully reverse even after iron is restored.
In pregnancy, untreated iron deficiency increases the risk of preterm birth, low birth weight, and postpartum complications. For anyone with a chronic condition that causes ongoing iron loss or poor absorption, the deficiency tends to recur without a plan to monitor and maintain iron stores over the long term.