Low hemoglobin happens when your body either can’t make enough red blood cells, destroys them too quickly, or loses them through bleeding. The normal range is 13.2 to 16.6 grams per deciliter for men and 11.6 to 15 for women. When your levels drop below those thresholds, a wide range of causes could be responsible, from something as fixable as a nutritional gap to something more complex like a chronic illness or bone marrow problem.
How Low Hemoglobin Feels
Mild drops in hemoglobin often cause no symptoms at all. As levels fall further, though, your body struggles to deliver enough oxygen to your tissues, and the effects become hard to ignore. The most common signs include persistent tiredness, weakness, shortness of breath, dizziness, cold hands and feet, and headaches. Some people notice pale or yellowish skin, an irregular heartbeat, or even chest pain. These symptoms tend to creep in gradually, which is why many people don’t realize their hemoglobin is low until a routine blood test catches it.
Iron Deficiency
Iron deficiency is the single most common reason for low hemoglobin worldwide. Your body needs iron to build heme, the oxygen-carrying core of each hemoglobin molecule. Inside developing red blood cells, iron gets inserted into a ring-shaped molecule called protoporphyrin to form heme. Without enough iron, that assembly line stalls, and your bone marrow produces smaller, paler red blood cells that carry less oxygen.
You can become iron-deficient in three main ways: not getting enough iron from food, not absorbing it well (common with celiac disease or after stomach surgery), or losing it through bleeding. Red meat, shellfish, beans, and fortified cereals are major dietary sources. Vegetarians and vegans are at higher risk because the type of iron in plant foods is harder for the body to absorb.
Blood Loss You Might Not Notice
Obvious blood loss from an injury or surgery is easy to connect to low hemoglobin. What catches many people off guard is slow, steady bleeding that happens internally. Peptic ulcers are the most common source of upper gastrointestinal bleeding. These open sores on the stomach lining or upper small intestine can seep small amounts of blood over weeks or months, draining your iron stores without producing visible symptoms. Tears in the esophageal lining, enlarged esophageal veins (often related to liver disease), and hemorrhoids are other frequent culprits.
Heavy menstrual periods are one of the leading causes of iron deficiency anemia in premenopausal women. If you regularly soak through a pad or tampon every hour or two, or your periods last longer than seven days, that blood loss adds up cycle after cycle.
Vitamin B12 and Folate Deficiency
Iron isn’t the only nutrient your body needs to build healthy red blood cells. Vitamin B12 and folate (vitamin B9) are both essential for red blood cell formation. When you’re short on either one, your bone marrow produces abnormally large cells called megaloblasts. These oversized cells don’t divide and reproduce the way normal red blood cells do, so fewer of them reach maturity. Many megaloblasts are too large to even leave the bone marrow and enter the bloodstream. The ones that do make it out die earlier than healthy red blood cells. The combined effect is a sharp drop in circulating red blood cells and, with it, hemoglobin.
B12 deficiency is especially common in older adults (who absorb it less efficiently), strict vegans (since B12 comes almost exclusively from animal products), and people with conditions that affect the stomach or small intestine. Folate deficiency often stems from a diet low in leafy greens, legumes, and fortified grains, or from heavy alcohol use.
Chronic Disease and Inflammation
Long-term illnesses like rheumatoid arthritis, lupus, inflammatory bowel disease, chronic infections, and cancer can lower hemoglobin even when your diet is perfectly fine. The mechanism is different from a simple nutritional shortfall. When your body is fighting ongoing inflammation, immune cells release signaling molecules that ramp up production of a hormone called hepcidin in the liver. Hepcidin acts like a gatekeeper: it blocks iron absorption from your gut and prevents your recycling system from releasing iron stored in immune cells back into the bloodstream.
The result is that iron gets locked away in storage, unavailable for hemoglobin production. At the same time, those same inflammatory signals reduce your kidneys’ output of erythropoietin, the hormone that tells bone marrow to make more red blood cells. So you end up with less raw material and a weaker production signal, both at once. This type of anemia typically improves only when the underlying disease is brought under control.
Kidney Disease
Healthy kidneys are the body’s main source of erythropoietin (EPO), the hormone that drives red blood cell production. As kidney function declines in chronic kidney disease, EPO output drops, and the bone marrow simply doesn’t get the message to produce enough red blood cells. Anemia is extremely common in people with advanced kidney disease and is one of the reasons kidney patients often feel exhausted and short of breath even before they need dialysis.
Bone Marrow Problems
Your bone marrow is the factory where all blood cells are made. When that factory is damaged or taken over by abnormal cells, hemoglobin production suffers directly.
In aplastic anemia, the stem cells that give rise to blood cells are injured or destroyed. The bone marrow still produces some cells, and those cells are healthy, but there simply aren’t enough of them. In myelodysplastic syndromes, damaged stem cells produce blood cells that are defective and don’t function properly, leaving you short on working red blood cells. Leukemia and other blood cancers can crowd the marrow with malignant cells, squeezing out the normal stem cells that would otherwise be making hemoglobin-rich red blood cells.
Red Blood Cells Breaking Down Too Fast
Sometimes the problem isn’t production at all. Your body makes red blood cells at a normal rate, but they’re being destroyed faster than they can be replaced. This is called hemolytic anemia, and it has several possible triggers. Autoimmune disorders can cause your immune system to mistakenly tag your own red blood cells as foreign and destroy them. Mechanical damage from faulty or artificial heart valves can physically shear red blood cells apart as blood flows through the heart. Certain infections, medications, and toxins can also accelerate red blood cell destruction.
Inherited Hemoglobin Disorders
Some people are born with genes that produce abnormal or insufficient hemoglobin. In sickle cell disease, a single amino acid change in the hemoglobin molecule dramatically alters the behavior of red blood cells, causing them to become rigid and crescent-shaped. These misshapen cells break down prematurely and clog small blood vessels.
Thalassemias work differently. Normal hemoglobin requires equal numbers of two protein chains, alpha and beta. In thalassemia, mutations reduce or eliminate production of one chain. The unpaired chains cause problems on their own. In severe alpha thalassemia, excess beta chains clump together into nonfunctional groups of four that can’t carry oxygen. In severe beta thalassemia, unpaired alpha chains are rapidly destroyed, and red blood cells are damaged in the process. Either way, the imbalance leads to anemia that can range from barely noticeable to life-threatening depending on how many gene copies are affected.
Pregnancy
A dip in hemoglobin during pregnancy is so predictable it has its own name: physiologic anemia of pregnancy. It isn’t caused by a problem with red blood cell production. Instead, blood plasma volume expands dramatically during pregnancy, especially in the third trimester, to support the growing fetus and placenta. Red blood cell mass increases too, but not as fast as plasma volume. The result is a dilution effect: the same number of red blood cells is swimming in a larger pool of fluid, so hemoglobin concentration measured per unit of blood drops.
This dilution is normal and expected, but it can mask true iron or folate deficiency, which is also common during pregnancy because the developing baby draws heavily on the mother’s nutrient stores. That overlap makes it important to distinguish between a harmless dilution effect and a genuine deficiency that needs treatment.