Summary
Haemoglobin is the iron-containing protein inside red blood cells that carries oxygen from the lungs to every tissue in the body. Low haemoglobin — anaemia — is one of the most common medical conditions globally, affecting an estimated 2 billion people. It causes fatigue, breathlessness, poor concentration, and pallor. Identifying the cause of anaemia (iron, B12, folate, chronic disease, or haemolysis) is essential for targeted treatment.
Each red blood cell contains approximately 270 million haemoglobin molecules, each capable of carrying four oxygen molecules. Haemoglobin synthesis depends on adequate iron, vitamin B12, folate, and erythropoietin (produced by the kidneys).
The most common cause of low haemoglobin globally is iron deficiency — particularly in premenopausal women and those with dietary restrictions. In the UK, iron deficiency anaemia affects approximately 3% of men and 8% of premenopausal women. Vitamin B12 and folate deficiency cause macrocytic anaemia (large, fewer red cells); iron deficiency causes microcytic anaemia (small, pale red cells).
Haemoglobin is part of the full blood count (FBC) — the most commonly requested blood test in the UK. It is interpreted alongside MCV, MCH, white cells, and platelets for complete haematological assessment.
What It Is
Haemoglobin (Hb) is a tetrameric metalloprotein consisting of four globin chains (typically two alpha and two beta in adult HbA), each wrapped around a haem group containing an iron atom. Oxygenation of haemoglobin (forming oxyhaemoglobin) and release of oxygen in tissues depends on the allosteric Bohr effect.
Haemoglobin production requires: (1) iron — for haem synthesis; (2) vitamin B12 and folate — for DNA synthesis in red cell precursors; (3) erythropoietin — the kidney-derived hormone that stimulates red cell production; (4) adequate globin chain synthesis.
Reference ranges differ by sex and altitude: men 130–170 g/L, women 120–160 g/L (UK). Anaemia is defined by WHO as Hb < 130 g/L in men and < 120 g/L in non-pregnant women.
Functions
Oxygen transport
Each haemoglobin molecule carries up to four oxygen molecules from the lungs to tissues — the core function of the red blood cell.
Carbon dioxide removal
Haemoglobin also transports approximately 20% of carbon dioxide from tissues back to the lungs for exhalation.
Anaemia diagnosis
Low haemoglobin defines anaemia — the most common blood disorder globally. Identifying the type guides targeted treatment.
Exercise capacity indicator
Haemoglobin directly determines oxygen-carrying capacity and therefore maximum aerobic exercise capacity (VO₂ max).
Reference Ranges
Haemoglobin
Measured in g/L| Status | Range (g/L) | Range (g/dL) | What it means |
|---|---|---|---|
| Low (Anaemia) | < 120 (women) / < 130 (men) | < 12 / < 13 g/dL | Anaemia — oxygen delivery to tissues is impaired. Investigate the cause. |
| Normal | 120–160 (women) / 130–170 (men) | 12–16 / 13–17 | Adequate oxygen-carrying capacity. |
| High | > 160 (women) / > 170 (men) | > 16 / > 17 | Polycythaemia — investigate for dehydration, chronic hypoxia, or polycythaemia vera. |
Reference ranges are sex-specific. Pregnancy lowers haemoglobin physiologically due to haemodilution. Athletes may have slightly higher haemoglobin. High altitude residents have physiologically elevated levels.
Symptoms of Imbalance
Anaemia symptoms correlate with severity and speed of onset — gradual anaemia is often remarkably well tolerated.
- Fatigue and low energy — even mild anaemia causes significant tiredness
- Breathlessness on exertion
- Pallor — pale skin, pale conjunctivae and inner eyelids
- Rapid heartbeat (palpitations)
- Dizziness and lightheadedness, especially on standing
- Cold hands and feet
- Difficulty concentrating and brain fog
- Ruddy (plethoric) complexion
- Headaches and dizziness
- Itching after a hot bath (polycythaemia vera)
- Blurred vision
- Risk of blood clots — deep vein thrombosis, stroke
Causes of Imbalance
- Iron deficiency (most common globally) — blood loss, poor intake, malabsorption
- Vitamin B12 deficiency
- Folate deficiency
- Anaemia of chronic disease (chronic inflammation, cancer, CKD)
- Haemolytic anaemia (red cell destruction)
- Bone marrow failure (aplastic anaemia, leukaemia)
- Renal failure (reduced erythropoietin)
- Dehydration (haemoconcentration)
- Polycythaemia vera (myeloproliferative disorder)
- Secondary polycythaemia: chronic hypoxia (COPD, high altitude), sleep apnoea
- Erythropoietin-secreting tumours
- Testosterone therapy
FAQs
Symptom onset depends on how quickly haemoglobin falls. Gradual anaemia (developing over weeks to months) is often remarkably well tolerated — some patients with Hb of 70–80 g/L experience only mild fatigue. Acute anaemia (from bleeding) causes severe symptoms at higher levels (< 100 g/L). Most people notice fatigue and breathlessness when Hb falls below 100 g/L; significant pallor and cardiovascular symptoms typically occur below 80 g/L.
Anaemia is classified by MCV (red cell size): microcytic (MCV 100 fL) — vitamin B12 or folate deficiency, alcohol, hypothyroidism. Identifying the type is the critical first step, as iron supplementation for B12-deficiency anaemia could mask neurological damage.
Reference ranges are based on population percentiles and reflect biological variation, not individual optimal values. Some individuals are constitutionally at the lower end of the normal range. However, a haemoglobin below the reference range always warrants investigation — even mild anaemia significantly impacts quality of life, exercise capacity, and cognitive performance.
Yes. Iron-rich foods (red meat, liver, dark leafy greens, legumes), vitamin B12 (meat, fish, eggs, dairy), and folate (leafy greens, legumes, fortified foods) are all required for normal haemoglobin production. Vitamin C alongside iron-rich plant foods dramatically improves non-haem iron absorption. Vegans are at higher risk of B12 and iron deficiency — both common causes of anaemia.
A slightly low haemoglobin warrants investigation — starting with ferritin (iron stores), vitamin B12, folate, and CRP. The cause determines the treatment: iron-deficiency anaemia responds to iron supplementation plus identifying the source of iron loss (particularly gastrointestinal bleeding in older adults); B12 deficiency requires B12 supplementation or injections; anaemia of chronic disease requires treating the underlying condition.
References
- WHO. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. WHO/NMH/NHD/MNM/11.1. 2011. View source
- Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832–1843. View source
- Cappellini MD, Motta I. Anemia in clinical practice — definition and classification. Semin Hematol. 2015;52(4):261–269. View source