Hemoglobin is the iron-containing protein inside red blood cells that carries oxygen from the lungs to every organ and tissue in the body. When hemoglobin levels fall below the normal range, the condition is called anemia. Low hemoglobin is one of the most common laboratory findings worldwide, it can range from a mild, easily corrected deficiency to a sign of significant blood loss or a chronic underlying condition. This guide explains what low hemoglobin means, the most common causes, and what to expect in terms of next steps.

What Is the Normal Range for Hemoglobin?

Normal hemoglobin levels vary by sex, age, and laboratory reference range. General guidelines:

• Adult men: 13.5-17.5 g/dL
• Adult women: 12.0-15.5 g/dL
• Pregnant women: 11.0 g/dL or above (WHO threshold; adjusted for trimester)
• Children: Varies by age, a provider or laboratory will specify the appropriate range

Severity of anemia by hemoglobin level (approximate, in non-pregnant adults):

• Mild: 10.0, lower limit of normal
• Moderate: 8.0-9.9 g/dL
• Severe: below 8.0 g/dL

Common Symptoms of Low Hemoglobin

The symptoms of anemia depend on the severity and speed of onset. Chronic, slowly developing anemia is often surprisingly well-tolerated. Symptoms include: fatigue and weakness (most common); pale skin, gums, and inner eyelids; shortness of breath with exertion; rapid or irregular heartbeat; dizziness or lightheadedness; headache; cold hands and feet; difficulty concentrating. Severe anemia can cause chest pain, fainting, and heart failure in extreme cases.

Causes of Low Hemoglobin

1. Iron-Deficiency Anemia (Most Common)

What it is: The most common cause of anemia worldwide, insufficient iron to produce adequate hemoglobin.

Why it happens: Iron deficiency occurs when the body's iron intake or absorption doesn't meet demand. Key causes: heavy menstrual bleeding; inadequate dietary iron; poor iron absorption (celiac disease, post-gastric surgery); chronic blood loss (GI bleeding from ulcers, polyps, or colorectal cancer); pregnancy (dramatically increased iron demands).

Lab clues: Low hemoglobin, low MCV (small red cells), low ferritin (the most sensitive marker for iron stores), low serum iron, high TIBC (total iron-binding capacity).

Next steps: Identify and treat the source of iron loss. Start iron supplementation as directed. In women with heavy periods or anyone with GI blood loss, investigate the underlying cause, GI bleeding prompts colonoscopy in adults over 45.

2. Vitamin B12 or Folate Deficiency

What it is: Deficiency of vitamins essential for red blood cell DNA synthesis, causing production of large, abnormal red cells (macrocytic anemia).

Why it happens: B12 deficiency, from pernicious anemia (autoimmune), veganism, poor absorption (chronic acid suppressor use, metformin, gastric surgery). Folate deficiency, from inadequate dietary intake, alcohol use, pregnancy, or medications like methotrexate.

Lab clues: Low hemoglobin, high MCV (large red cells), low serum B12 or folate, elevated homocysteine.

Next steps: Supplement with the deficient vitamin. B12 deficiency from pernicious anemia may require intramuscular injections since oral absorption is impaired. Folate deficiency responds well to oral supplementation and dietary improvement.

3. Anemia of Chronic Disease (ACD)

What it is: A common anemia associated with chronic inflammatory conditions (rheumatoid arthritis, IBD, chronic infections, cancer) that impairs red blood cell production and iron utilization.

Why it happens: Inflammatory cytokines increase hepcidin, a hormone that blocks iron release from stores, while also suppressing bone marrow red cell production. Iron may be "normal" or even elevated in stores but unavailable for use.

Lab clues: Mild to moderate anemia, often normal or slightly low MCV, elevated ferritin (stores are adequate but unavailable), elevated inflammatory markers (CRP, ESR).

Next steps: Treating the underlying chronic disease is the primary approach. Iron supplementation alone is often ineffective because the issue is iron utilization, not iron stores.

4. Chronic Kidney Disease (CKD) Anemia

What it is: Kidneys produce erythropoietin (EPO), a hormone that stimulates red blood cell production. Damaged kidneys produce insufficient EPO, causing anemia.

Why it happens: As kidney function declines (measured by eGFR), EPO production falls. Reduced EPO means less stimulation for bone marrow to make red cells.

Lab clues: Anemia alongside elevated creatinine and reduced eGFR; typically normocytic (normal MCV); ferritin and B12 usually normal.

Next steps: EPO-stimulating agents (ESAs) are used to treat anemia of CKD in appropriate patients. Iron supplementation is often co-prescribed since effective erythropoiesis requires adequate iron.

5. Hemolytic Anemia

What it is: Red blood cells are being destroyed faster than the bone marrow can replace them.

Why it happens: Autoimmune hemolysis (the immune system attacks red cells), hereditary conditions (sickle cell disease, thalassemia, G6PD deficiency), mechanical destruction, or medications can all cause hemolysis.

Lab clues: Low hemoglobin, elevated bilirubin and LDH (breakdown products of hemolysis), low haptoglobin, elevated reticulocyte count (bone marrow trying to compensate), spherocytes or abnormal red cell shapes on blood smear.

Next steps: Hematology referral for evaluation and treatment. The approach varies significantly by cause.

6. Blood Loss (Acute or Chronic)

What it is: Direct loss of red blood cells through bleeding, either a single significant episode or slow ongoing blood loss.

Why it happens: GI bleeding (ulcers, colorectal cancer, hemorrhoids), post-surgical blood loss, trauma, or heavy menstrual periods are common sources. Chronic GI blood loss is a frequent and underrecognized cause of iron-deficiency anemia in adults over 50.

Lab clues: Acute loss, normal MCV initially, dropping rapidly. Chronic loss, typically develops iron-deficiency pattern over time (low MCV, low ferritin).

Next steps: Identify and stop the source of bleeding. GI blood loss in adults over 45 warrants colonoscopy to exclude colorectal cancer.

When to Seek Medical Care

Any confirmed low hemoglobin should be evaluated by a healthcare provider to identify the cause.

Call 911 or go to the emergency room immediately if you have:

• Chest pain or pressure
• Severe shortness of breath, or shortness of breath at rest
• Fainting or loss of consciousness
• A rapid or irregular heartbeat along with chest pain, dizziness, sweating, or nausea

These symptoms can signal that the heart is being critically strained and need emergency evaluation, not urgent care, especially if hemoglobin is below 7-8 g/dL or dropping quickly.

Contact your doctor promptly (same-day or urgent care) if you have:

• New or worsening fatigue
• Mild palpitations without chest pain, fainting, or shortness of breath at rest
• Pale skin, dizziness, or lightheadedness that isn't severe

Related Articles:
Understanding a Complete Blood Count (CBC) | Causes of Fatigue | Causes of Shortness of Breath | Iron Deficiency Anemia in Pregnancy