Hemoglobin Blood Test (HGB): Normal Range, Low & High

Hemoglobin — reported on your lab sheet as HGB, Hgb, or Hb (all three mean the same thing) — is the protein inside your red blood cells that carries oxygen through your body. The hemoglobin blood test sits at the heart of the complete blood count (CBC) and is the single key marker of anemia. Low hemoglobin defines anemia; high hemoglobin, which is less common, is called polycythemia. This guide explains normal hemoglobin levels for men, women, children, and pregnancy, what causes low hemoglobin and high hemoglobin, the symptoms of each, and when a result is worth raising with your doctor — with iron deficiency being by far the most common cause of a low result.

Key takeaways

• Hemoglobin (HGB/Hgb/Hb) carries oxygen; it is measured on every complete blood count (CBC).¹
• Anemia = low hemoglobin. U.S. surveillance uses < 13.0 g/dL in males 15 and older and < 12.0 g/dL in females 12 and older; the WHO threshold in pregnancy is < 11 g/dL.²³
• Anemia is the most common blood disorder, affecting about 3 million Americans; in recent national data, 9.3% of people age 2+ were anemic — 13.0% of females vs 5.5% of males.⁴²
• The number-one cause of anemia worldwide is iron deficiency, followed by B12/folate deficiency, chronic inflammation, kidney disease, hemolysis, and hemoglobin disorders (thalassemia, sickle cell disease).⁵⁶
• MCV (red cell size) points to the cause: microcytic anemia (iron), macrocytic (B12/folate), normocytic (inflammation, kidney disease…).⁷⁸
• High hemoglobin usually comes from dehydration, smoking, altitude or low oxygen — and, rarely, a bone-marrow disease (polycythemia vera, JAK2 mutation).⁹
• You don't raise hemoglobin at random: you treat the cause, once it has been identified.

What is hemoglobin?

Hemoglobin is an iron-rich protein packed inside your red blood cells (erythrocytes). It picks up oxygen in the lungs and delivers it to your organs — which is exactly why fatigue and shortness of breath appear when it runs low. Its concentration in blood is a direct reflection of how much oxygen your blood can carry.¹

You'll see it abbreviated several ways, and they all refer to the same measurement: HGB (the form most analyzers and U.S. lab reports print), Hgb, and Hb (the form used most often in medical literature). There is no difference between them — only house style.

Hemoglobin is measured on the complete blood count (CBC), alongside the red blood cell count, the hematocrit, and the red cell indices — above all the MCV (mean corpuscular volume), the average size of your red cells, which is a valuable clue to the cause of an anemia. Hemoglobin is therefore always read with the rest of the CBC and, very often, with iron studies (iron, ferritin, transferrin saturation).¹⁰

Why is hemoglobin measured?

Hemoglobin appears on nearly every blood draw. Clinicians order it to:¹⁴

• investigate fatigue, pale skin, shortness of breath, dizziness, headaches, or hair loss;
• screen for or monitor anemia (iron deficiency, pregnancy, chronic disease);
• follow kidney disease, cancer, chemotherapy, or an inflammatory condition;
• assess bleeding or check a patient before surgery;
• confirm eligibility to donate blood, where a minimum hemoglobin is required.

Normal hemoglobin levels

Below are indicative reference values for adults. They vary with sex, age, pregnancy, race, and the laboratory's method — MedlinePlus notes explicitly that the reference range differs by lab, age, race, and sex, so always compare your result to the range printed on your report.¹

Good to know: in the United States hemoglobin is reported in g/dL (grams per deciliter). Some countries report g/L — to convert, divide by 10 (130 g/L = 13 g/dL). The thresholds that define anemia were updated by the WHO in 2024 and vary by age and pregnancy status; U.S. national surveillance applies the same age- and sex-specific cutoffs across race and ethnicity groups.³² What matters clinically is the size of the drop, how fast it happened, and the MCV — not the number alone.

Reading hemoglobin with the rest of your CBC

Hemoglobin tells you whether you're anemic. The red cell indices tell you why:

• MCV — the average size of your red cells. Low (microcytic), normal (normocytic), or high (macrocytic) splits anemia into three diagnostic families.
• MCHC — the average hemoglobin concentration inside each cell, i.e. how pale or color-packed they are. Small + pale is the classic iron-deficiency fingerprint.
• RDW — how much your red cells vary in size. A high RDW often rises early in iron deficiency and helps separate it from thalassemia trait.

Interpreting your results

Low hemoglobin: anemia

A low hemoglobin is the definition of anemia. It is the most common blood disorder in the country, affecting about 3 million Americans, and it is far more frequent in women: national data from 2021–2023 put anemia at 9.3% of people age 2 and older — 13.0% of females versus 5.5% of males — with prevalence rising as income falls.⁴²

To find the cause, clinicians start with the MCV:⁷

• Microcytic anemia (small cells, low MCV) → most often iron deficiency, the leading cause worldwide; confirmed with ferritin and iron studies.⁵ Less commonly, thalassemia.¹²
• Macrocytic anemia (large cells, high MCV) → vitamin B12 or folate (B9) deficiency.⁷¹³
• Normocytic anemia (normal-sized cells) → chronic inflammation, kidney disease, acute bleeding, or hemolysis (red cell destruction); a reticulocyte count then shows whether the marrow is still regenerating.⁶

What causes low hemoglobin

The most frequent causes fall into four groups:⁵¹¹

• Blood loss — heavy menstrual periods, gastrointestinal bleeding, surgery, trauma. This is the cause that must never be missed in an unexplained anemia in men or postmenopausal women.
• Not enough intake or absorption — a diet low in iron, B12, or folate; celiac disease; bariatric surgery; some medications.¹³
• Chronic disease — inflammation, chronic kidney disease (where the kidneys make less erythropoietin), cancer, chemotherapy, autoimmune conditions.⁶⁴
• Red cell production or destruction problems — sickle cell disease and thalassemia, hemolytic anemias, bone marrow disorders.¹⁴¹²

In children, the evaluation follows the same logic but with age-specific thresholds, with iron deficiency again leading the list.⁸

Symptoms of low hemoglobin include fatigue, pale or yellow skin, shortness of breath on exertion, palpitations or an irregular heartbeat, dizziness, weakness, and headaches. Their intensity depends on how deep the anemia is and how quickly it developed — a slow decline is often surprisingly well tolerated, which is why anemia is frequently discovered on a routine blood test rather than because of symptoms.⁴

How do you raise hemoglobin? By treating the cause: iron (oral first, sometimes intravenous) for iron deficiency, B12 or folate for a vitamin deficiency, and management of the underlying disease. You should not supplement blindly — someone with normal iron does not need iron, and an unexplained anemia needs to be worked up, sometimes to look for a source of bleeding.⁵

High hemoglobin: polycythemia

A high hemoglobin (polycythemia, or erythrocytosis) is much less common than a low one. Most of the time it is "relative" — you are dehydrated, so the blood is concentrated and hemoglobin only looks high. The other frequent explanations are chronic low oxygen, which pushes the body to make more red cells: smoking, lung disease, sleep apnea, some heart disease, and living at high altitude.¹

Much more rarely, a high hemoglobin reflects polycythemia vera, a bone-marrow disease associated with the JAK2 mutation. It is considered in particular above roughly 16.5 g/dL in men or 16.0 g/dL in women when other signs are present, and it is confirmed with JAK2 testing rather than the CBC alone.⁹ Testosterone therapy and some medications can also raise hemoglobin.

A high hemoglobin that persists on a repeat test — once dehydration has been ruled out — should be explained by your physician, because true polycythemia raises the risk of clotting and is treatable.⁹

What can affect your hemoglobin

Several non-disease factors move hemoglobin and are worth mentioning to your clinician:¹

• hydration — dehydration falsely raises it; overhydration falsely lowers it;
• pregnancy — hemoglobin normally falls through dilution as plasma volume expands;
• altitude — people living at high elevation run higher hemoglobin;
• smoking, which raises it;
• sex and age, and race, all of which shift the reference range;¹
• recent blood donation or transfusion;
• medications and supplements;
• the lab's method and analyzer, which is why ranges differ between laboratories.

When to see a doctor

Contact your clinician if your hemoglobin is outside your lab's range — especially if it is falling over time, markedly low, or paired with symptoms such as fatigue, pallor, shortness of breath, palpitations, or dizziness. A low hemoglobin normally leads to iron studies, B12 and folate levels, and a look for a source of bleeding; a confirmed high hemoglobin is evaluated for smoking, sleep apnea, lung disease, and — if unexplained — polycythemia vera.¹⁰⁹ Seek prompt care for chest pain, fainting, severe breathlessness, or visible heavy bleeding.

Recent research

According to PubMed and clinical-trial registries:

• Iron deficiency, with or without anemia. A landmark review recalls that iron deficiency affects ~2 billion people; oral iron remains first-line, with intravenous iron reserved for intolerance, malabsorption, chronic inflammation, or advanced pregnancy.⁵ Trials are currently comparing oral and IV iron in heart failure (for example IRONICA, NCT07053475).¹⁵ (Auerbach M et al., JAMA, 2025 — DOI.)
• Anemia of kidney disease: oral drugs. A newer class, the HIF prolyl-hydroxylase inhibitors (for example roxadustat, now approved), raises hemoglobin by mouth, comparably to erythropoietin injections.¹⁶¹⁷ (Zheng Q et al., Pharmacological Research, 2020 — DOI.)
• Anemia thresholds under review. The WHO revised its hemoglobin cutoffs defining anemia in 2024, by age and pregnancy status — a framework labs are progressively adopting.³ The question of whether thresholds should differ by ancestry remains open: an NHANES III follow-up found that the hemoglobin level associated with excess mortality in older adults differed by race/ethnicity, being lower in non-Hispanic Black participants.¹⁸ (Patel KV et al., British Journal of Haematology, 2009 — DOI.) U.S. national surveillance still applies uniform age- and sex-specific cutoffs across groups.²

These findings concern diagnosis and management; they do not authorize self-medication and do not replace your physician's advice.

Get your hemoglobin interpreted by AI DiagMe

Hemoglobin is never read alone: its meaning depends on the rest of your CBC (MCV, MCHC, RDW, red cell count), on your iron and ferritin, on your B12 and folate, and on your context. That cross-referencing is what gives the number its real value.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

Bottom line

Hemoglobin (HGB) carries oxygen, and a drop in it defines anemia. Remember the thresholds — < 13.0 g/dL in men, < 12.0 g/dL in women, < 11.0 g/dL in pregnancy — that iron deficiency is the number-one cause, and that MCV points to the type of anemia (iron, B12/folate, inflammation). A high hemoglobin is most often dehydration, but is worth confirming. You don't raise it at random: you treat the cause. No value is read in isolation — what counts is all your markers together with your profile, which is exactly what AI DiagMe provides, alongside your physician.

Sources

Official sources and peer-reviewed publications (PubMed, ClinicalTrials.gov, ChEMBL) used for this guide: