Low hemoglobin means your blood is carrying less oxygen than your body needs. For men, the healthy range is 13.2 to 16.6 grams per deciliter (g/dL). For women, it’s 11.6 to 15 g/dL. Falling below these ranges is called anemia, and the cause usually falls into one of three categories: your body isn’t making enough red blood cells, it’s losing them through bleeding, or it’s destroying them too quickly.
What Low Hemoglobin Feels Like
The symptoms of low hemoglobin are easy to dismiss because they overlap with so many other things. Fatigue is the hallmark, but not the ordinary end-of-day tiredness. It’s the kind where you feel wiped out after climbing a single flight of stairs or can’t concentrate at work no matter how much sleep you got. Shortness of breath during mild activity, dizziness or lightheadedness when standing, pale skin, and cold hands and feet are all common. Some people notice a yellowish tint to their skin, which signals that red blood cells are being broken down faster than normal.
The tricky part is that hemoglobin can drop gradually over weeks or months, and your body adapts along the way. You might not realize how bad you feel until treatment brings your levels back up and you suddenly remember what normal energy feels like. Symptoms generally become more noticeable once hemoglobin dips below 10 g/dL, and hospitals typically consider a blood transfusion when levels fall to 7 or 8 g/dL.
Iron Deficiency: The Most Common Cause
Iron deficiency is responsible for more cases of low hemoglobin than any other single cause. Your body needs iron to build a component called heme, which is assembled inside the energy-producing centers of your cells (the mitochondria). Heme is the part of hemoglobin that actually grabs onto oxygen. Without enough iron, your bone marrow still tries to produce red blood cells, but they come out smaller than normal and carry less oxygen per cell.
Women with heavy menstrual periods are especially vulnerable because they lose iron-rich blood on a regular cycle. Pregnancy is another major factor. During pregnancy, blood volume expands significantly to support the growing baby, which dilutes the concentration of hemoglobin even when red blood cell production increases. This dilution effect is greatest in women carrying large babies or multiples. On top of that, the developing baby draws on the mother’s iron stores, which can push a borderline deficiency into full-blown anemia.
Diet plays a role too, particularly for vegetarians and vegans, since the form of iron in plant foods is harder for the body to absorb than the form found in meat. But diet alone rarely causes severe iron deficiency in otherwise healthy adults eating enough calories. When iron levels are very low in a man or a postmenopausal woman, doctors look for hidden blood loss rather than assuming it’s dietary.
Slow Bleeding You Might Not Notice
One of the most important causes of low hemoglobin is blood loss that happens so gradually you never see it. The digestive tract is the usual source. Peptic ulcers, which are open sores in the stomach or upper small intestine, are the most common cause of upper gastrointestinal bleeding. Colon polyps (small growths on the lining of the colon), hemorrhoids, and tears in the esophagus can also bleed in amounts too small to change the color of your stool but large enough to drain your iron stores over time.
In more serious cases, tumors in the esophagus, stomach, colon, or rectum can weaken the digestive tract lining and cause chronic bleeding. This is one reason doctors take unexplained iron-deficiency anemia seriously, especially in adults over 50. A colonoscopy or upper endoscopy is often part of the workup, not because cancer is the most likely explanation, but because it’s the one you don’t want to miss.
Vitamin B12 and Folate Deficiency
Iron isn’t the only nutrient your body needs to build healthy red blood cells. Vitamin B12 and folate are both essential for cell division. When either is low, your bone marrow produces red blood cells that are abnormally large and don’t function properly. This is a useful diagnostic clue: if your blood test shows that your red blood cells are larger than normal, the cause is more likely a B12 or folate problem than an iron problem. Small red blood cells, on the other hand, point toward iron deficiency or certain inherited conditions.
B12 deficiency is common in older adults because the stomach produces less of the acid needed to absorb it from food. It’s also common in people who follow strict vegan diets, since B12 is found almost exclusively in animal products. Certain autoimmune conditions can destroy the stomach cells responsible for B12 absorption, leading to a specific form of anemia that used to be fatal before supplementation was available.
Chronic Disease and Kidney Problems
Many long-term illnesses cause low hemoglobin through mechanisms that have nothing to do with diet or blood loss. Chronic kidney disease is one of the clearest examples. Your kidneys produce a hormone that signals your bone marrow to make red blood cells. When the kidneys are damaged, they produce less of this hormone, so the bone marrow slows production. Less oxygen reaches your organs and tissues, and hemoglobin drops.
Chronic infections, autoimmune diseases like rheumatoid arthritis and lupus, and cancers can also suppress red blood cell production. The body’s inflammatory response essentially locks iron away in storage, keeping it out of the bone marrow even when your total iron levels are technically adequate. This type of anemia doesn’t respond well to iron supplements because the problem isn’t a lack of iron. It’s the body’s inability to use what it already has.
Red Blood Cells Breaking Down Too Early
Red blood cells normally live about 120 days before the body recycles them. In hemolytic anemia, that lifespan is cut short. The spleen and liver clear damaged or abnormal cells from the bloodstream, and if destruction outpaces production, hemoglobin falls.
The causes split into two broad groups. Some are inherited: conditions where the red blood cell itself is defective from the start. Others are acquired, meaning something outside the cell is attacking it. On the inherited side, sickle cell disease produces misshapen hemoglobin that distorts the cell into a rigid crescent shape, and thalassemia causes the body to make less hemoglobin overall, resulting in fragile, undersized cells. Both conditions are more common in people of African, Mediterranean, Middle Eastern, and Southeast Asian descent.
Acquired causes include autoimmune reactions where your immune system mistakenly targets your own red blood cells, certain medications, infections (malaria is the most significant worldwide), and mechanical damage from defective heart valves or other devices in the bloodstream. Even severe infections with common bacteria can trigger red cell destruction through toxins. If your skin or eyes have a yellowish tint alongside low hemoglobin, it’s a strong signal that red blood cells are being broken apart, since the yellow color comes from a pigment released during that breakdown.
How Doctors Find the Cause
A standard blood count (CBC) does more than just measure your hemoglobin level. It also measures the size of your red blood cells, which is one of the fastest ways to narrow down the reason for anemia. Smaller-than-normal cells suggest iron deficiency or thalassemia. Larger-than-normal cells point toward B12 or folate deficiency, or sometimes liver disease. Normal-sized cells with low hemoglobin can indicate chronic disease, kidney problems, or early blood loss before iron stores have been fully depleted.
From there, your doctor may check iron levels, B12, folate, kidney function, or markers of inflammation depending on what the initial results suggest. If iron deficiency is confirmed but the cause isn’t obvious (like heavy periods or a clearly inadequate diet), the next step is usually looking for a source of hidden bleeding, particularly in the digestive tract. In cases where red blood cell destruction is suspected, specialized tests can detect antibodies attached to red blood cells or abnormal hemoglobin types.
The key point is that low hemoglobin is always a symptom, not a diagnosis. The number on your lab report tells you something is off, but figuring out why requires following the trail of clues your blood work provides. The cause determines the treatment, and treating the wrong cause (like taking iron supplements when the real problem is B12 deficiency) won’t fix anything.