Iron deficiency is a condition where your body’s iron reserves are too low to support normal functions, most critically the production of hemoglobin, the protein in red blood cells that carries oxygen. It affects roughly 1.27 billion people worldwide, making it the most common nutritional deficiency on the planet. Iron deficiency can exist on a spectrum, from mildly depleted stores with no obvious symptoms to full-blown anemia where red blood cell production is severely impaired.
What Iron Actually Does in Your Body
About two-thirds of your body’s iron sits inside hemoglobin, where it picks up oxygen in the lungs and delivers it to every tissue and organ. A separate protein called myoglobin uses iron to store and supply oxygen specifically to working muscles. Beyond oxygen transport, iron plays roles you might not expect. It’s essential for producing thyroid hormones, which regulate your metabolism. It supports enzymes involved in energy production and DNA repair. In children, adequate iron is directly tied to cognitive development, school performance, and normal behavior patterns.
When iron runs low, none of these processes work properly. Your cells get less oxygen, your muscles fatigue faster, your brain works less efficiently, and even your thyroid can slow down.
How Deficiency Develops in Three Stages
Iron deficiency doesn’t happen overnight. It progresses through three distinct stages, and understanding them helps explain why you can be iron deficient long before you feel obviously sick.
In the first stage, your body’s stored iron starts to drop, but your red blood cells are still functioning normally. You likely won’t notice anything. In the second stage, stores are low enough that your bone marrow begins producing red blood cells without enough hemoglobin inside them. You may start to feel run down, but a standard blood count might still look borderline. In the third stage, hemoglobin falls below the normal range and you’ve crossed into iron deficiency anemia, where symptoms become harder to ignore.
Recognizing the Symptoms
The early signs of iron deficiency are easy to dismiss because they overlap with so many other things: fatigue, weakness, difficulty concentrating, feeling cold when others don’t. As deficiency worsens, more specific symptoms appear. Skin may look noticeably pale, especially around the inner eyelids and nail beds. You might get headaches, feel dizzy when standing, or notice your heart racing during light activity.
Some symptoms are distinctive enough to point directly at iron deficiency. Koilonychia, or “spoon nails,” is one of the more recognizable signs. Your nails gradually flatten, then develop a concave dip deep enough to hold a drop of water. Pica, an unusual craving for non-food items like ice, dirt, or starch, is another hallmark. Some people develop a sore, swollen tongue or cracks at the corners of their mouth. Restless legs, a creeping urge to move your legs especially at night, is also closely linked to low iron.
Common Causes
Iron deficiency comes down to a simple imbalance: your body is either not getting enough iron, losing too much of it, or unable to absorb what you take in.
Blood loss is the leading cause in adults. For people who menstruate, heavy periods are the most frequent culprit. Internal bleeding you can’t see, from ulcers, polyps, or other gastrointestinal issues, is another major source. Frequent blood donation can also deplete stores over time.
Inadequate dietary intake is especially common in restrictive diets. Your gut absorbs heme iron (from meat, poultry, and seafood) at a rate of 25 to 30 percent, while non-heme iron from plant foods like beans, lentils, and spinach is absorbed at only 3 to 5 percent. That’s a 200 to 400 percent difference in bioavailability, which means vegetarians and vegans need to be more intentional about their iron sources.
Absorption problems account for another category. Celiac disease, inflammatory bowel conditions, and surgeries that bypass or remove portions of the small intestine can all interfere with your body’s ability to pull iron from food. Your body also regulates iron absorption through a hormone called hepcidin. When you’re healthy, hepcidin rises and falls to keep iron levels balanced. But chronic inflammation can push hepcidin too high, effectively locking iron out of your bloodstream even when your diet is adequate. In rare genetic cases, hepcidin stays permanently elevated, causing a form of iron deficiency that doesn’t respond to oral supplements.
Pregnancy creates a surge in iron demand. The growing fetus, expanding blood volume, and placenta all require extra iron, which is why deficiency during pregnancy is so common without supplementation.
Who Is Most at Risk
Children under five have the highest rates of iron deficiency of any age group. Women between ages 10 and 60 consistently show higher prevalence than men, driven by menstruation and pregnancy. Men over 80 are an often-overlooked group where rates climb steeply, likely due to chronic disease and reduced dietary intake. Globally, iron deficiency cases grew from roughly 985 million in 1990 to over 1.27 billion in 2021, and projections estimate nearly 1.44 billion affected by 2050.
How Iron Deficiency Is Diagnosed
A standard blood count can show anemia, but ferritin, a protein that reflects your stored iron, is the most useful single test for identifying deficiency before it reaches the anemia stage. The World Health Organization recommends a ferritin below 15 micrograms per liter as the standard threshold for iron deficiency in otherwise healthy individuals. However, ferritin rises during infection and inflammation, which can mask true deficiency. In people with inflammatory conditions, a ferritin below 30 in children or below 70 in adults is considered a more reliable cutoff.
Your doctor may also check additional markers like transferrin saturation and total iron-binding capacity to get a fuller picture, especially if the initial results are ambiguous.
What Happens if It Goes Untreated
Mild iron deficiency that’s caught early is straightforward to correct. Left untreated, the consequences escalate. Prolonged anemia forces the heart to pump harder to compensate for reduced oxygen in the blood. Over time, this can lead to an enlarged heart, irregular heartbeat, or heart failure.
During pregnancy, untreated anemia raises the risk of preterm delivery, low birth weight, and anemia in the newborn, which can cause developmental problems. In children, even iron deficiency without full anemia has been associated with lasting effects on cognitive development and academic achievement.
Treatment and What to Expect
Oral iron supplements are the first-line treatment for most people. Therapeutic doses typically range from 100 to 200 milligrams of elemental iron per day, depending on how severe the deficiency is. The most commonly used formulations contain varying amounts of elemental iron per tablet, so the number of pills you take daily will depend on which type your provider recommends.
Side effects are common and worth knowing about in advance: nausea, constipation, stomach cramps, and dark stools. If you can’t tolerate a standard dose, taking a lower amount or dosing every other day can significantly reduce gut symptoms while still replenishing stores, though more slowly. Vitamin C taken alongside your supplement improves absorption. Calcium, tea, and coffee inhibit it, so spacing these away from your iron dose helps.
Replenishing iron stores is not a quick process. Most people need three to six months of consistent supplementation to fully rebuild reserves, even if symptoms start improving within a few weeks. For people who can’t absorb oral iron due to gut conditions or who need faster correction, intravenous iron is an alternative that bypasses the digestive system entirely.
Treating the deficiency itself is only half the equation. Identifying and addressing the underlying cause, whether that’s heavy menstrual bleeding, a gut absorption issue, or an inadequate diet, is what prevents the cycle from repeating.