Hemoglobin
Oxygen-carrying protein in red blood cells
Also known as
Definition
Hemoglobin is the iron-containing protein in erythrocytes that transports oxygen from the lungs to tissues. Low levels define anemia and may manifest as fatigue, pallor, and reduced physical performance. Hemoglobin is routinely measured in a complete blood count and is a core vital parameter for assessing oxygen-carrying capacity.
| Parameter | Value |
|---|---|
| Unit | g/dl |
| Reference Range | Women: 12.0–16.0 g/dl | Men: 13.5–17.5 g/dl |
Agree on your personal target range with your doctor.
↓ What a low value means
Hemoglobin below sex-specific reference values defines anemia (WHO: women < 12.0 g/dl, men < 13.0 g/dl). Mild anemia (Hb 10–12/13 g/dl): fatigue, pallor, reduced exercise capacity. Severe anemia (Hb < 8 g/dl): tachycardia, dyspnea, cognitive impairment. Causes: iron deficiency (most common worldwide), vitamin B12/folate deficiency, chronic disease, blood loss, hemolytic disorders.
↑ What a high value means
Elevated hemoglobin occurs in polycythemia vera (myeloproliferative), reactive polycythemia (altitude, smoking, COPD, sleep apnea), or doping (EPO). The value at which a result counts as elevated depends on the laboratory as well as on age and sex (MedlinePlus); the reference range on the report is what applies. Risks: elevated blood viscosity, thromboembolic hazard. Athletes at altitude may physiologically show elevated values.
✓ What can influence this value
The hemoglobin value is influenced by the supply of iron, vitamin B12, and folate, blood loss, chronic diseases, kidney function, and bone marrow diseases. Fluid balance, staying at high altitude, smoking, and pregnancy also change the value (hemoconcentration or dilution). Absorption of iron from food is promoted by vitamin C and inhibited by tea and coffee. The assessment is made by the treating doctor.
When to test
In the complete blood count (CBC), routinely determined during check-ups. No fasting required. The value is also measured during pregnancy and to monitor known anemia. How often it is measured is decided by your doctor.
Frequently asked questions
What is the difference between hemoglobin and ferritin? +
Ferritin reflects iron stores and falls before hemoglobin with deficiency. Hemoglobin only falls when stores are largely depleted. Low ferritin with normal hemoglobin points to latent iron deficiency without anemia, which can already cause symptoms such as fatigue.
How does anemia affect exercise capacity? +
Anemia can limit exercise capacity and maximal performance. With severe anemia, a racing heart and shortness of breath can occur. Whether and how much exercise is possible depends on the cause and severity and is decided by your doctor.
How quickly does hemoglobin rise after iron therapy? +
With iron therapy, hemoglobin usually rises within a few weeks; the iron stores (ferritin) refill more slowly. How long treatment lasts and when it is checked is decided by your doctor.
Hemoglobin
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.