What the hemoglobin test shows
Hemoglobin is the iron-containing protein inside red blood cells that binds oxygen in the lungs and carries it to every tissue. The test measures how much sits in a set volume of blood — the blood’s oxygen-carrying capacity, as MedlinePlus puts it. It is the single number doctors use to define anemia.
It is read with the other red-cell measures but differs from them. Hematocrit is the percentage of blood volume made up of red cells and the red blood cell count is their number; both track hemoglobin but can diverge when cells are unusually small or large. Ferritin is different again — it measures stored iron, the reserve spent before hemoglobin falls, so it flags a shortfall earlier.
Hemoglobin normal range
Hemoglobin is reported in g/dL (United States) or g/L (most of the world); the two differ only by a factor of ten (13 g/dL = 130 g/L). Typical adult orientation ranges:
| Group | Typical reference, g/dL (= g/L) | Anemia below (WHO) |
|---|---|---|
| Men (adult) | ~13.5–17.5 (135–175) | 13.0 (130) |
| Women (adult, non-pregnant) | ~12.0–15.5 (120–155) | 12.0 (120) |
| Pregnancy | slightly lower than non-pregnant | 11.0 (110) first trimester; 10.5 (105) later |
| Children and teenagers | age-specific — use your lab’s range | varies with age |
The right-hand column is the WHO 2024 cutoff for anemia: below 13 g/dL (130 g/L) in men and 12 g/dL (120 g/L) in non-pregnant women. In pregnancy the 2024 guideline uses trimester-specific cutoffs — below 11 g/dL (110 g/L) in the first trimester and 10.5 g/dL (105 g/L) in the second and third. High altitude and smoking genuinely raise hemoglobin and are adjusted for before a level is called low. Reference ranges depend on the lab, sex and age — always read your result against your own report.
Why hemoglobin is low
Low hemoglobin means anemia — a fall in hemoglobin, hematocrit or red-cell count, per StatPearls — and it is far more common than a high value. Roughly by frequency:
- Iron deficiency (commonest worldwide). Usually blood loss — heavy periods in younger women, or in men and postmenopausal women a slow gut bleed (ulcer, polyp or colorectal cancer) until proven otherwise. Low intake and malabsorption (celiac disease, H. pylori) also count. Stores fall first, so ferritin confirms it.
- Chronic disease and inflammation: infection, autoimmune conditions, cancer and chronic kidney disease, where the kidney makes less erythropoietin.
- Vitamin B12 or folate deficiency, which produces oversized red cells.
- Acute blood loss from trauma, surgery or a gut bleed.
- Other causes: hemolysis, an underactive thyroid, bone-marrow disorders, and inherited conditions such as thalassemia or sickle cell disease.
Symptoms — fatigue, breathlessness on exertion, pallor and dizziness — overlap with thyroid problems, so iron deficiency and hypothyroidism are easily confused in tired women and hemoglobin is central to any work-up for persistent tiredness. When is it urgent? Severe anemia with chest pain, marked breathlessness, fainting or a racing heart needs same-day care, and black or bloody stools point to active bleeding.
Why hemoglobin is high
A high hemoglobin (erythrocytosis) is less common and often not a blood disease. Roughly by frequency:
- Dehydration — the commonest reason. Less plasma concentrates the red cells, so hemoglobin reads high with no real increase; it corrects with fluids.
- Living at high altitude, a normal adaptation to thinner air.
- Chronic low oxygen: long-term smoking, COPD and other lung disease, and obstructive sleep apnea, which push the kidney to make more erythropoietin.
- Testosterone or anabolic-steroid use, and erythropoietin (EPO) doping.
- Polycythemia vera, a bone-marrow disorder (usually the JAK2 mutation) that overproduces red cells — and often platelets and white blood cells too.
The level that triggers a polycythemia vera work-up is hemoglobin above 16.5 g/dL (165 g/L) in men or 16.0 g/dL (160 g/L) in women, or a raised hematocrit, as StatPearls sets out; a ruddy face, headaches or itching after a hot shower are clues. When is it urgent? A very high hemoglobin thickens the blood and raises clot, heart-attack and stroke risk, so persistent high values — or a severe headache, blurred vision or a suspected clot — need prompt hematology review.
What to test alongside
Hemoglobin is one line of the complete blood count; read it with the rest plus a few add-ons:
- Hematocrit and red blood cells — move with hemoglobin; red-cell indices (MCV) classify the anemia as small-, normal- or large-cell.
- Ferritin — iron stores; falls before hemoglobin, so it confirms or rules out iron deficiency.
- CRP and ESR — inflammation that can lower hemoglobin (anemia of chronic disease).
- TSH — an underactive thyroid causes anemia and the same fatigue.
- Creatinine — kidney function; chronic kidney disease is a common cause of anemia.
- White blood cells and platelets — often raised with hemoglobin in polycythemia vera.
What to do about an abnormal result
- Don’t self-treat. Do not start iron on your own — it will not fix a B12, kidney or thyroid cause and can mask a bleed that needs finding, as NICE stresses.
- Read it in context. One value can mislead, so it is interpreted with the whole blood count and your symptoms, and repeated when you are well and hydrated if needed.
- For low hemoglobin: your doctor looks for the cause — ferritin and iron studies, B12 and folate, thyroid and kidney tests, and a search for blood loss — and treats it; unexplained deficiency may prompt a gastroenterology, gynecology or hematology referral.
- For high hemoglobin: first checks are hydration, smoking, and lung and sleep history; a persistently high level goes to hematology, often with JAK2 testing for polycythemia vera.
- See your GP first; they coordinate the next step. Urgent symptoms — chest pain, fainting, a suspected clot or heavy bleeding — need same-day or emergency care.
Mini-FAQ
What is a normal hemoglobin level?
Adult men run roughly 13.5–17.5 g/dL (135–175 g/L) and non-pregnant women about 12.0–15.5 g/dL (120–155 g/L), but every lab prints its own range. The WHO calls it anemia below 13 g/dL (130 g/L) in men and 12 g/dL (120 g/L) in women.
What does low hemoglobin mean?
It means anemia — too little oxygen-carrying capacity in the blood. Iron deficiency from blood loss is the commonest cause; others include B12 or folate deficiency, chronic inflammation, kidney disease and an underactive thyroid.
How is hemoglobin different from ferritin?
Hemoglobin is the iron already carrying oxygen inside red blood cells; ferritin is stored iron held in reserve. Because stores empty first, ferritin falls before hemoglobin, flagging an iron shortfall earlier.
When is a high hemoglobin dangerous?
A mildly high value is often just dehydration, high altitude or smoking. A persistently high level — above about 16.5 g/dL in men or 16.0 g/dL in women — needs a work-up for polycythemia vera, because thickened blood raises the risk of clots and stroke.
