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Hemoglobin

Hemoglobin is the oxygen-carrying protein in your red blood cells and the marker that defines anemia.

What is Hemoglobin?

Haemoglobin is the iron-containing protein inside your red blood cells. Each molecule binds oxygen in the lungs and delivers it to your tissues. Your haemoglobin concentration is literally your blood's oxygen-carrying capacity. It is the marker that defines anaemia. On its own, haemoglobin tells you how much capacity you have — not why. To find the cause, read it alongside the red cell indices (MCV, MCH, MCHC, RDW) and your iron status (ferritin, transferrin saturation) — and where relevant vitamin B12 and folate. The pattern across those markers points toward the cause.

Why is Hemoglobin relevant?

Haemoglobin determines how much oxygen your blood transports — and you feel it. When it falls far enough, the familiar signals follow: fatigue, breathlessness on exertion, lightheadedness, and slower recovery. A stable haemoglobin is foundational for energy and endurance. Haemoglobin and iron status move on different timelines. Ferritin (your iron store) falls first; haemoglobin holds up until the deficiency is well advanced. A normal haemoglobin does not rule out an early iron shortfall. Always read them together.

Hemoglobin high or low — what it means

Read haemoglobin alongside hematocrit, MCV, MCH, MCHC, and RDW. Small cells point toward iron deficiency; large cells toward B12 or folate deficiency; normal cells toward chronic disease or recent blood loss. The pattern, not the single number, points toward the cause. Hydration affects the value: dehydration concentrates your blood and can make haemoglobin look artificially high. Measure when well hydrated and not right after heavy training. Confirm a low haemoglobin with a repeat test and follow up with ferritin, transferrin saturation, and where needed B12 — do not start supplementation blindly.

Hemoglobin reference ranges

MenAbout 13.5-17.5 g/dL (NHG reference, adult, non-pregnant)8.5-11 mmol/L
WomenAbout 12-16 g/dL (NHG reference, adult, non-pregnant)7.5-10 mmol/L
Anemia cut-off men (WHO)International WHO anemia threshold for men<13 g/dL
Anemia cut-off women (WHO)WHO threshold for non-pregnant women<12 g/dL

Cut-offs vary by lab, method and guideline. Dutch labs usually report in mmol/L, international labs in g/dL (roughly 15 g/dL = 9.3 mmol/L). Ranges fall in pregnancy and shift further by trimester. Not a diagnosis - always read haemoglobin together with iron status and the red cell indices.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →Read the guide: Energy →

Frequently asked questions

What is a normal hemoglobin level?

As a general reference (lab-dependent, not a diagnosis), Dutch primary-care guidance (NHG) puts adults outside pregnancy at roughly 8.5–11 mmol/L for men (≈ 13.5–17.5 g/dL) and 7.5–10 mmol/L for women (≈ 12–16 g/dL). Ranges fall in pregnancy because of natural blood dilution, and the cut-off shifts further by trimester. Always check which unit (mmol/L or g/dL) your report uses before comparing values, and read hemoglobin as a trend across several measurements.

What does a low hemoglobin mean?

A hemoglobin below the lower bound counts as anemia: too little oxygen-carrying capacity. Internationally, the WHO sets its anemia cut-off at below 13 g/dL in men and below 12 g/dL in non-pregnant women. A low hemoglobin most often traces back to iron deficiency (low intake, poor absorption, or blood loss such as heavy periods or gut bleeding), to B12 or folate deficiency, to chronic inflammation or kidney disease, or to recent blood loss. The right next step is to confirm it on a repeat test and check ferritin, transferrin saturation, and where indicated B12 and folate — rather than reaching for supplements blindly.

What does a high hemoglobin mean?

A high hemoglobin is more often dehydration or, in athletes, training and altitude adaptation than a real problem — hemoglobin is a concentration, so a smaller plasma volume can make it look artificially high. A persistently and genuinely high value (sometimes linked to smoking, sleep apnea, lung conditions, or rarer marrow disorders) warrants a clinician's assessment. Measure well hydrated and under comparable conditions, and lean on the trend rather than a single snapshot.

What symptoms point to low hemoglobin, and when is it concerning?

When hemoglobin falls far enough, the classic signs can follow: fatigue, breathlessness on exertion, paleness, lightheadedness, a faster heartbeat, and slower recovery. Never read hemoglobin in isolation — read it alongside hematocrit and the red cell indices (MCV, MCH, MCHC, RDW) and your iron status, because the pattern, not the single number, points toward the cause. A result below the lower bound, persistent symptoms, or an unexplained value is worth assessing with a healthcare professional.

Related biomarkers

Hematocrit
Hematocrit is the percentage of your blood made up of red blood cells, your measure of oxygen-carrying capacity.
Erythrocytes (RBC Count)
Erythrocytes are your red blood cells, and the RBC count measures them per volume of blood to gauge oxygen transport.
MCV
MCV is the average size of your red blood cells and helps distinguish the type of anemia.
RDW
RDW shows how much your red blood cells vary in size from one another.

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