What it measures.
Hemoglobin is the iron-containing protein within red blood cells that binds oxygen in the lungs and delivers it throughout the body. Low hemoglobin (anemia) reduces oxygen delivery, causing fatigue and weakness; high levels may indicate dehydration or polycythemia.
The concentration of hemoglobin protein in blood, measured in g/dL. Each hemoglobin molecule contains four iron atoms that reversibly bind oxygen.
Why it matters.
Hemoglobin determines your blood's oxygen-carrying capacity. Even mild anemia reduces exercise tolerance and energy. Identifying and correcting the underlying cause improves quality of life and prevents complications.
Physiology.
Red blood cells are produced in bone marrow and filled with hemoglobin. Each RBC contains ~280 million hemoglobin molecules. Hemoglobin's iron binds oxygen in the oxygen-rich lungs and releases it in oxygen-poor tissues. RBCs circulate for ~120 days before recycling.
Testing & preparation.
How to prepare
- No special preparation needed
- Part of standard CBC (complete blood count)
- Hydration status can affect results
When to test
Part of routine health screening, evaluating fatigue, or monitoring known anemia.
How often
Annually as part of routine labs; more frequently during treatment for anemia.
Interpretation.
High hemoglobin
Common causes:
- Dehydration (concentrated blood)
- Polycythemia vera
- Living at high altitude
- Chronic lung disease (compensatory)
- Testosterone therapy
- Performance-enhancing drug use
Implications:
- Increased blood viscosity
- Higher thrombosis risk (stroke, heart attack)
- May indicate underlying condition
- Polycythemia requires investigation
Low hemoglobin
Common causes:
- Iron deficiency (most common)
- B12 or folate deficiency
- Chronic disease anemia
- Blood loss (acute or chronic)
- Bone marrow disorders
- Hemolysis (RBC destruction)
- Kidney disease (reduced EPO)
Implications:
- Fatigue, weakness, exercise intolerance
- Shortness of breath
- Pale skin, cold hands/feet
- Dizziness, lightheadedness
- Rapid or irregular heartbeat
- Reduced quality of life
Optimization.
Diet
- Iron-rich foods: red meat, poultry, fish, legumes
- Vitamin C with plant iron sources to enhance absorption
- B12 from animal products or supplements
- Folate from leafy greens and legumes
Lifestyle
- Identify and address blood loss sources
- Avoid blood donation if anemic
- Regular moderate exercise (stimulates RBC production)
- Address underlying conditions
Supplements
- Iron only if deficient (ferritin <50 ng/mL)
- B12 if deficient or vegan
- Folate if deficient
- Vitamin C to enhance iron absorption
FAQs.
How long does it take to correct anemia?
Timeline depends on the cause. Iron deficiency: hemoglobin typically rises within 2-3 weeks of starting supplements, with full correction in 6-8 weeks. However, continue iron 4-6 months to replenish stores. B12 deficiency: responds within weeks with injections or high-dose oral supplements. Chronic disease anemia: requires treating the underlying condition.
Can I have symptoms even with 'normal' hemoglobin?
Yes. A hemoglobin at the lower end of normal (e.g., 12.0 g/dL in a woman) may represent a significant drop from your personal baseline. Athletes and active individuals often notice symptoms earlier because of higher oxygen demands. Also, iron deficiency causes fatigue before hemoglobin falls—check ferritin if symptoms persist with normal hemoglobin.