A low hemoglobin (Hgb) result means your blood isn’t carrying as much oxygen as it should. The normal range is 13.2 to 16.6 g/dL for men and 11.6 to 15 g/dL for women, so anything below those thresholds shows up as “low” on your lab report. The most common reason is iron deficiency, but several other conditions can drive hemoglobin down, and figuring out which one applies to you usually requires a few more blood tests.
How Hemoglobin Works
Hemoglobin is a protein packed inside every red blood cell. Its job is to pick up oxygen in your lungs and deliver it to every tissue in your body. The protein is built from two main components, alpha and beta chains, and each one holds an iron atom at its center. That iron is what actually binds to oxygen. Without enough iron, or without enough properly built hemoglobin, oxygen delivery drops and your body starts showing the effects.
Iron Deficiency: The Most Common Cause
Iron deficiency accounts for the majority of low hemoglobin cases worldwide. Your bone marrow needs iron to build hemoglobin, and when iron stores run low, production slows down. This doesn’t happen overnight. First, your stored iron (measured by a blood test called ferritin) gradually depletes. Your gut compensates by absorbing more iron from food, but if intake can’t keep up with demand, your marrow eventually can’t make enough red blood cells. That’s when your hemoglobin drops.
The reason your iron is low matters just as much as the number itself. Common culprits include:
- Heavy menstrual periods. Losing a large volume of blood each month is one of the most frequent causes of iron deficiency in premenopausal women.
- Slow bleeding you can’t see. A stomach ulcer, a polyp in the colon, or regular use of aspirin and other pain relievers can cause small, ongoing blood loss in the digestive tract. You may not notice it at all, but over weeks and months it drains your iron stores.
- Not enough iron in your diet. This is more common in people who eat very little red meat, are pregnant, or are going through rapid growth (like adolescents).
- Poor absorption. Conditions like celiac disease or prior stomach surgery can prevent your gut from pulling iron out of food efficiently.
If your doctor suspects iron deficiency, they’ll typically order ferritin levels and a test called iron-binding capacity to confirm it. If the cause of iron loss isn’t obvious, they may recommend checking for hidden bleeding in the stool.
Vitamin B12 and Folate Deficiency
Your body also needs vitamin B12 and folate (vitamin B9) to produce red blood cells properly. When either is missing, the bone marrow produces abnormally large red blood cells that don’t function well. This is called megaloblastic anemia. The red blood cells are bigger than normal but fewer in number, and each one carries less effective hemoglobin.
B12 deficiency is common in people over 50 (because stomach acid production drops with age, and you need acid to absorb B12), in strict vegans (since B12 comes almost exclusively from animal products), and in people with autoimmune conditions that attack the stomach lining. Folate deficiency tends to show up in people with poor diets, heavy alcohol use, or certain digestive conditions. Both are straightforward to test for with a blood draw.
Chronic Disease and Inflammation
Ongoing inflammation in the body can suppress hemoglobin production even when your iron and vitamin levels look reasonable. This type of anemia shows up alongside a wide range of chronic conditions: rheumatoid arthritis, lupus, inflammatory bowel disease (Crohn’s or ulcerative colitis), chronic kidney disease, diabetes, heart failure, cancer, and long-term infections like HIV or tuberculosis.
The mechanism is different from iron deficiency. Inflammatory signals tell your body to lock iron away in storage cells rather than releasing it to the bone marrow. Your ferritin level might actually look normal or high, even though your marrow is starved for usable iron. This is why doctors sometimes order additional tests to distinguish inflammation-related anemia from true iron deficiency, since the treatments are different.
Blood Loss You Might Not Expect
Any ongoing blood loss lowers hemoglobin over time. Beyond heavy periods and GI bleeding, less obvious sources include frequent blood donations, surgical blood loss that hasn’t fully recovered, and nosebleeds that happen regularly. Cancer in the digestive tract can also cause slow, hidden bleeding that presents as unexplained low hemoglobin long before other symptoms appear. This is one reason doctors take a persistently low Hgb seriously, especially in older adults or anyone with new, unexplained iron deficiency.
Inherited Conditions
Some people have low hemoglobin because of the way their body is genetically programmed to build it. Thalassemia is the most common inherited cause. It occurs when the genes responsible for making the alpha or beta parts of hemoglobin are missing or altered. The result is fewer functional red blood cells that also have shorter lifespans. People with thalassemia trait (carrying one affected gene) often have mildly low hemoglobin their entire lives and may not know why until they see it on a blood test.
Sickle cell disease is another inherited condition that affects hemoglobin structure, causing red blood cells to become rigid and break down faster than normal. Both thalassemia and sickle cell disease are identified through specific hemoglobin analysis tests.
Bone Marrow Problems
Less commonly, the bone marrow itself is the issue. In aplastic anemia, the marrow is damaged and stops producing enough blood cells of all types, not just red blood cells. Myelodysplastic syndromes are another group of rare disorders where the marrow makes defective blood cells. These conditions are serious but uncommon, and they typically cause abnormalities across multiple blood cell lines (white cells and platelets in addition to red cells), which your doctor can spot on a standard complete blood count.
What Low Hemoglobin Feels Like
The symptoms of low hemoglobin are all tied to reduced oxygen delivery. Fatigue is the hallmark, often described as a tiredness that sleep doesn’t fix. You may feel short of breath during activities that didn’t used to wind you, notice your heart beating faster than usual, or feel dizzy when you stand up. Pale skin, cold hands and feet, and brittle nails are other common signs. Mild drops in hemoglobin sometimes cause no symptoms at all, which is why it’s often caught on routine bloodwork.
The severity of symptoms generally tracks with how low the hemoglobin is and how quickly it dropped. A slow, gradual decline gives your body time to adapt, so you might feel relatively normal even at levels that would flatten someone whose hemoglobin dropped suddenly.
What Happens After a Low Result
A single low hemoglobin number doesn’t tell the whole story. Your doctor will look at the size and shape of your red blood cells on the complete blood count, which narrows down the type of anemia. Small red blood cells point toward iron deficiency or thalassemia. Large red blood cells suggest B12 or folate deficiency. Normal-sized red blood cells with low hemoglobin often indicate chronic disease or recent blood loss.
From there, follow-up tests are chosen based on the most likely cause. A reticulocyte count shows whether your bone marrow is trying to compensate by ramping up production. Ferritin, iron levels, B12, and folate help identify nutritional deficiencies. If hidden bleeding is suspected, a stool test or imaging of the digestive tract may be next. The goal is always to find and treat the underlying cause rather than just the number on the lab report.