Low iron means your body doesn’t have enough iron to carry out its normal functions, most critically delivering oxygen to your tissues. Iron is the core component of hemoglobin, the protein in red blood cells that picks up oxygen in your lungs and carries it throughout your body. When iron drops too low, your cells receive less oxygen, which is why fatigue is usually the first thing you notice. A ferritin level (a blood marker of stored iron) below 15 ng/mL is diagnostic of iron deficiency, while levels between 15 and 30 ng/mL suggest probable deficiency.
Why Iron Matters in Your Body
Iron sits at the center of hemoglobin and a related protein called myoglobin, which stores oxygen in your muscles. Each iron atom has a binding site specifically shaped to grab an oxygen molecule. When oxygen attaches, the iron atom physically shifts position inside the protein, locking the oxygen in place for transport. In your lungs, hemoglobin picks up oxygen; in your tissues, it releases it. Without enough iron, fewer red blood cells can be made, and the ones that exist are smaller and carry less oxygen.
Your body also uses iron for energy production inside cells, immune function, and brain signaling. This is why low iron causes problems that go well beyond feeling tired. In children under two, iron deficiency can cause irreversible effects on brain development, affecting learning and school performance later in life. A mother’s iron status during the last trimester of pregnancy also influences her baby’s cognitive development.
How Iron Deficiency Develops
Iron deficiency doesn’t happen overnight. It progresses in stages. First, your stored iron (ferritin) drops as your body draws on its reserves. At this point you may feel fine because your hemoglobin is still normal. Next, the iron available for making new red blood cells starts running low, a stage sometimes called iron-deficient erythropoiesis. Finally, hemoglobin itself falls, and you develop iron deficiency anemia, the most advanced form.
Many people sit in the early or middle stages for months or years without realizing it. Symptoms can be subtle at first and easy to dismiss as stress or poor sleep.
Common Causes of Low Iron
The three main reasons iron drops are blood loss, poor absorption, and not getting enough from food.
Blood Loss
This is the most common cause in adults. Heavy menstrual periods are the leading driver in premenopausal women. Slow, hidden bleeding inside the digestive tract is another major source. Peptic ulcers, hiatal hernias, colon polyps, colorectal cancer, and inflammatory bowel diseases like ulcerative colitis and Crohn’s disease can all cause chronic low-grade blood loss you might not notice. Regular use of over-the-counter pain relievers like aspirin, ibuprofen, and naproxen can also cause internal bleeding over time. Frequent blood donation raises your risk too.
Poor Absorption
Iron from food enters your bloodstream through the small intestine. Celiac disease, Crohn’s disease, and ulcerative colitis can all impair this process. Gastric bypass and other weight-loss surgeries that remove or bypass sections of the small intestine also reduce iron absorption significantly.
Your body regulates iron absorption through a hormone called hepcidin. When hepcidin levels rise, it blocks the only known channel that moves iron from your gut into your blood. This is relevant because inflammation, infection, and even high single doses of iron supplements can spike hepcidin, temporarily shutting down absorption.
Increased Demand or Low Intake
Pregnancy dramatically increases iron needs. Without supplementation, many pregnant people develop iron deficiency anemia. Vegetarians and vegans are also at higher risk because plant-based iron is harder for the body to absorb than iron from meat. Diets low in iron-rich foods (red meat, shellfish, beans, fortified cereals, dark leafy greens) can gradually deplete stores over months.
Symptoms to Recognize
Early iron deficiency often causes no obvious symptoms. As stores drop further, you may experience persistent fatigue, weakness, pale skin, cold hands and feet, brittle nails, and headaches. Shortness of breath during activities that used to feel easy is common once anemia develops.
Some symptoms are less well known. Restless legs syndrome, an overwhelming urge to move your legs at night accompanied by tingling, crawling, or aching sensations, is strongly linked to low iron. About four out of five people with restless legs also have involuntary leg movements during sleep, occurring every 20 to 40 seconds, which disrupts sleep quality for both the person and their partner.
Pica, an unusual craving for non-food items like ice, dirt, or starch, is another hallmark of iron deficiency. If you find yourself compulsively chewing ice, that’s worth mentioning to your doctor. Other signs include a sore or swollen tongue, cracks at the corners of your mouth, and frequent infections.
How Low Iron Is Diagnosed
A standard blood test can identify iron deficiency. The most useful single marker is ferritin, which reflects your iron stores. The diagnostic thresholds for adults are straightforward:
- Below 15 ng/mL: confirms iron deficiency
- 15 to 30 ng/mL: probable iron deficiency
- Above 30 ng/mL: iron deficiency is unlikely
- Above 100 ng/mL: normal iron stores
Your doctor may also check transferrin saturation, which measures how much of your blood’s iron-carrying protein is actually loaded with iron. Normal ranges are 20 to 50 percent for men and 15 to 50 percent for women. Values below these ranges suggest your body doesn’t have enough iron to go around. A complete blood count showing small, pale red blood cells points toward iron deficiency anemia specifically.
One important caveat: ferritin rises during inflammation and infection, so a “normal” ferritin reading can mask true iron deficiency if you have a chronic inflammatory condition. In those cases, transferrin saturation and other markers become more important.
Treating Low Iron
For most people, oral iron supplements are the first step. Traditional recommendations call for 150 to 200 mg of elemental iron per day, split across multiple doses. But recent research using isotope tracking has changed that thinking. Studies in iron-deficient women found that doses of 60 mg or more trigger a spike in hepcidin that lasts about 24 hours, effectively blocking further absorption until it subsides. By 48 hours, hepcidin returns to baseline.
This means taking iron every other day, as a single morning dose, actually results in better absorption than daily divided doses. A practical approach based on this research: 60 to 120 mg of elemental iron taken in the morning on alternate days. This schedule also tends to cause fewer side effects like nausea, constipation, and stomach pain, which are the main reasons people stop taking iron.
Vitamin C improves iron absorption, and not just when taken alongside iron-rich food. Research shows vitamin C improves iron status in women even when consumed separately from iron sources, likely because it suppresses hepcidin production. Calcium, polyphenols (found in tea and coffee), and phytic acid (found in whole grains and legumes) reduce absorption, so it helps to take iron supplements away from these.
Addressing the underlying cause matters as much as supplementation. If heavy periods are driving the deficiency, managing menstrual bleeding is part of the solution. If celiac disease is impairing absorption, treating that condition comes first. For people who can’t tolerate oral iron or who have severe anemia, intravenous iron is an alternative that bypasses the gut entirely and replenishes stores faster.
How Long Recovery Takes
You may start feeling better within a week or two of starting iron supplements, but rebuilding your stores takes much longer. Hemoglobin levels typically improve over six to eight weeks. Full replenishment of ferritin stores usually requires three to six months of continued supplementation. Stopping too early is one of the most common mistakes, because hemoglobin normalizes well before your reserves are restored, leaving you vulnerable to a quick relapse.