Iron deficiency is the most common nutritional deficiency worldwide, occurring when your body’s iron stores drop too low to support normal red blood cell production and tissue function. Nearly 31% of women aged 15 to 49 and roughly 40% of children under five are affected globally, most often without realizing it. The condition develops gradually through distinct stages, starting long before anemia shows up on a blood test.
How Iron Deficiency Develops in Stages
Iron exists in three compartments in your body: stored iron (mainly in the liver), transport iron (circulating in the blood), and functional iron (actively working inside red blood cells and tissues). Deficiency progresses as each compartment empties in sequence.
The first stage is iron depletion, where your stored reserves run low but your body can still produce red blood cells normally. You likely feel fine at this point, and a standard blood count may look completely normal. The only clue is a dropping ferritin level, the protein that reflects how much iron you have banked away.
The second stage is iron-deficient erythropoiesis, a term that simply means your bone marrow is starting to struggle. There still isn’t enough hemoglobin loss to flag anemia, but the supply chain is strained. Markers like transferrin saturation (how much of your iron-carrying protein is actually loaded with iron) begin to fall below 16%.
The third and most severe stage is iron deficiency anemia, where hemoglobin concentration drops measurably. This is when symptoms become hard to ignore and when most people finally get diagnosed. By this point, the deficiency has been building for weeks or months.
What Causes It
Iron deficiency boils down to three mechanisms: losing too much iron, not absorbing enough, or not eating enough. Often, more than one of these is happening at the same time.
Blood loss is the most common cause in adults. In premenopausal women, heavy menstrual periods are the leading driver. In older adults and men, the culprit is more often slow, hidden bleeding in the digestive tract from ulcers, esophagitis, or growths. Even hiatus hernias carry roughly a 20% incidence of iron deficiency anemia due to small ulcers forming where the stomach is compressed by the diaphragm. Hookworm infections remain a major cause in tropical regions, driving iron loss through intestinal inflammation.
Malabsorption is the second major category. Celiac disease is a prime example: iron deficiency anemia shows up in 30 to 70% of people with celiac disease and is sometimes the only detectable sign at diagnosis. When the lining of the upper small intestine is damaged, iron simply can’t cross into the bloodstream efficiently. Inflammatory bowel disease and gastric bypass surgery create similar problems.
Dietary insufficiency alone can also tip the balance, especially for vegetarians, vegans, and growing children. The body absorbs about 25% of heme iron (from meat, poultry, and fish) but 17% or less of non-heme iron (from plants, beans, and fortified grains). Compounds in tea, coffee, and dairy products further reduce non-heme absorption, while vitamin C counteracts those inhibitors. A person relying heavily on plant-based iron sources while drinking tea with meals may absorb only a small fraction of the iron on their plate.
Symptoms Beyond Fatigue
Fatigue and pallor are the textbook symptoms, but iron deficiency produces a surprisingly wide range of effects because iron is involved in oxygen transport, energy production, and brain chemistry.
Early on, you may notice that your endurance drops, that you feel winded doing things that used to be easy, or that you’re unusually cold. As the deficiency worsens, symptoms can include dizziness, headaches, brittle nails, and a sore or swollen tongue. Some people develop koilonychia, where fingernails become thin and scoop upward like a spoon.
Two less well-known symptoms deserve attention. Pica is an unusual craving for non-food items like ice, dirt, or starch. It’s surprisingly common in iron-deficient people and often resolves once iron levels are restored. Restless legs syndrome, that irresistible urge to move your legs especially at night, has a strong connection to iron status. One Korean study found restless legs in over 40% of patients with iron deficiency, and across Western European populations, an estimated 5 to 15% of people experience the condition, with iron deficiency detected in 25 to 44% of those cases.
Why It Matters Most During Pregnancy
About 35.5% of pregnant women worldwide are anemic, and iron deficiency is the leading cause. The stakes are higher during pregnancy because iron demands roughly double to support the growing placenta, expanding blood volume, and developing fetus.
Iron deficiency during the first trimester has a more negative impact on fetal growth than deficiency developing later. It’s linked to premature labor, low birth weight, and intrauterine growth restriction. For the mother, symptoms include palpitations, breathing difficulties, fainting, and sleep disruption, along with increased risk of preeclampsia and postpartum infection.
The effects on the baby extend beyond birth. Iron is essential for neural metabolism, neurotransmitter function, and myelination (the insulation that helps nerves conduct signals quickly). Infants born to iron-deficient mothers face higher risks of cognitive, language, and motor developmental delays that can persist into early childhood.
How It’s Diagnosed
Ferritin is the go-to blood test, but the thresholds used to define deficiency are more complicated than they appear. The WHO sets the cutoff at less than 15 μg/L for women and less than 12 μg/L for young children. The CDC uses 15 μg/L or less for anyone over six months. But a growing body of evidence suggests these thresholds are too low.
A 2025 multinational study published in The Lancet Global Health found that hemoglobin starts declining when ferritin drops below about 25 μg/L in women and 22 μg/L in children. Separate studies using the body’s own iron-absorption signals as a guide found that the body begins compensating for low iron at ferritin levels around 40 to 50 μg/L. In other words, your body recognizes a shortage well before current diagnostic cutoffs would flag one.
Ferritin also has a major blind spot. It rises during any kind of inflammation because it’s an acute-phase reactant, meaning infections, autoimmune conditions, liver disease, or even a bad cold can push ferritin levels up and mask a true deficiency. For people with chronic conditions like inflammatory bowel disease, kidney disease, or heart failure, a ferritin below 100 μg/L or a transferrin saturation below 20% is considered diagnostic for iron deficiency. If ferritin falls between 100 and 300 μg/L in these patients, transferrin saturation is needed to confirm whether iron is genuinely low.
How Iron Is Replaced
Oral iron supplements are the standard first step. A common therapeutic dose for adults is 150 to 200 mg of elemental iron per day, though this is considerably more than what the body actually needs to absorb. Because only about 10% of supplemental iron is absorbed, that large dose delivers roughly 15 to 20 mg to the body. For context, the daily requirement is about 10 mg of elemental iron for men and postmenopausal women, 20 mg for premenopausal women, and 30 mg during pregnancy.
Side effects like nausea, constipation, and stomach cramps are common at higher doses. Taking iron every other day rather than daily has gained traction as an approach that may improve absorption efficiency while reducing gut symptoms. Pairing your supplement with a source of vitamin C and avoiding tea, coffee, or dairy at the same time makes a meaningful difference in how much iron actually gets through.
For people who can’t tolerate oral iron or who have absorption problems (celiac disease, inflammatory bowel disease, prior gastric surgery), intravenous iron is an alternative that bypasses the gut entirely. Replenishing iron stores typically takes three to six months of consistent supplementation, and ferritin should be rechecked to confirm that stores have actually rebuilt rather than simply stopping when symptoms improve.