The oxygen-carrying protein in red blood cells; central to energy, breathlessness, and anaemia assessment.
What it is
Haemoglobin (Hb) is the iron-rich protein packed inside red blood cells. Each molecule can bind oxygen in the lungs and release it in tissues that need fuel for energy production. UK laboratories almost always report haemoglobin in grams per litre (g/L), not the older g/dL scale you may see in overseas reports or older textbooks.
A haemoglobin result is one line on a full blood count (FBC). It is read with haematocrit, red-cell count, and red-cell indices (MCV, MCH, MCHC, RDW), and often with iron studies or vitamin B12/folate when anaemia is suspected. Sex-specific adult reference intervals are standard in UK practice because typical values differ between adult men and women.
Each haemoglobin tetramer carries up to four O₂ molecules. UK g/L reporting means an older “13.5 g/dL” is 135 g/L, check units when comparing overseas results or wearable app guesses.
Why it matters
Low haemoglobin is the laboratory definition of anaemia. Common contributors include iron deficiency (dietary, menstrual blood loss, gut losses), vitamin B12 or folate deficiency, long-term illness, kidney disease, and less often bone-marrow or inherited disorders. Symptoms can include tiredness, shortness of breath on exertion, pale skin, and reduced exercise tolerance, but many people feel surprisingly well until levels fall further.
High haemoglobin can reflect dehydration (concentrated blood), smoking, living or training at altitude, testosterone therapy, or true over-production of red cells (polycythaemia). A single raised value after a hard race or with poor fluid intake should not be over-interpreted without clinical context.
Lifestyle context that often matters: dietary iron and vitamin C with plant iron sources; menstrual history; gut symptoms or black stools (seek urgent care if bleeding is suspected); alcohol; and any medicines that affect bleeding or marrow. Do not start high-dose iron or other supplements solely from one result without advice: excess iron can be harmful.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
When haemoglobin is adequate, tissues receive a steady oxygen supply for mitochondria to make ATP, the energy currency of muscle and brain. When it falls, the heart and breathing rate often compensate, which is why anaemia feels like fitness has vanished even if training has not changed.
Haemoglobin also buffers blood pH and carries a small amount of carbon dioxide. Chronically low oxygen delivery can affect mood, concentration, cold tolerance, and wound healing. Chronically high haemoglobin thickens blood (higher viscosity), which can strain the circulation and raise clotting risk in susceptible people, another reason unexplained high values need clinical review.
For athletes
For endurance athletes, haemoglobin (with ferritin and iron studies) is one of the most watched “oxygen delivery” markers. Low-normal or falling Hb can explain heavy legs, flat workouts, and slower recovery even before frank anaemia. Female athletes with heavy periods and athletes in energy deficit are at higher risk of iron-related falls.
Hard training, racing, and foot-strike haemolysis can nudge red-cell turnover; altitude camps raise Hb physiologically. Acute dehydration after long sessions can falsely raise Hb, compare like-with-like (hydration, time of day). Do not chase “high Hb” with self-prescribed agents; unexplained rises need medical assessment. Pair interpretation with ferritin, MCV, and how training load feels.
