Hemoglobin is one of the first values a doctor checks when reviewing blood test results. It shows how effectively the blood is delivering oxygen to the body's tissues. This guide explains normal hemoglobin levels, what affects them, and what abnormal values mean.
What Is Hemoglobin
Hemoglobin (HGB) is an iron-containing protein inside red blood cells. Its job is to bind oxygen in the lungs, carry it to tissues throughout the body, and transport carbon dioxide back.
The hemoglobin level reflects how well this process works. When levels drop, tissues receive less oxygen — leading to the classic symptoms of anemia.
Normal Hemoglobin Levels — Chart by Group
Normal values depend on sex, age, and physiological state.
| Group | Normal Hemoglobin (g/L) |
|---|---|
| Women 18–65 | 120–148 |
| Men 18–65 | 130–160 |
| Women over 65 | 117–145 |
| Men over 65 | 125–155 |
| Pregnant women | 110–140 |
| Children 1–6 years | 110–140 |
| Children 7–14 years | 115–145 |
Men naturally have higher hemoglobin levels due to the influence of testosterone on red blood cell production and greater average muscle mass.
Causes of Low Hemoglobin
Low hemoglobin is called anemia — a condition, not a diagnosis in itself. The underlying cause varies.
Iron deficiency is the most common cause worldwide. Iron is a core component of hemoglobin; without enough of it, the body cannot produce sufficient hemoglobin. It occurs with poor diet, chronic blood loss (heavy periods, peptic ulcer), or impaired absorption.
Vitamin B12 and folate deficiency — these vitamins are essential for red blood cell maturation. Their deficiency causes megaloblastic anemia.
Chronic diseases — kidney disease, chronic inflammation (rheumatoid arthritis, Crohn's disease), and cancer can suppress red blood cell production.
Blood loss — acute (trauma, surgery) or chronic (GI bleeding).
Bone marrow disorders — less common, but possible causes of impaired blood cell production.
Causes of High Hemoglobin
Elevated hemoglobin is less common but worth investigating.
- Dehydration — when blood plasma volume decreases, hemoglobin concentration rises. The most common and benign cause.
- Smoking — carbon monoxide from cigarettes reduces oxygen delivery, prompting the body to produce more hemoglobin.
- High altitude living — a normal physiological adaptation to lower atmospheric oxygen.
- Chronic lung disease (COPD, sleep apnea) — the same compensatory mechanism.
- Polycythemia vera — a rare blood disorder where bone marrow overproduces red blood cells.
Symptoms of Hemoglobin Abnormalities
When hemoglobin is low:
- persistent fatigue and weakness
- pale skin and mucous membranes
- dizziness, especially when standing up
- shortness of breath with normal activity
- rapid or irregular heartbeat
- headache
- brittle nails and hair loss (with iron deficiency)
When hemoglobin is high:
- facial flushing
- headache and pressure
- increased blood viscosity → elevated clotting risk
- skin itching after hot showers (characteristic of polycythemia vera)
How to Prepare for the Test
Hemoglobin is measured as part of a standard CBC. For accurate results:
- Fast for 8–12 hours before the test. Water is fine.
- Avoid intense exercise the day before.
- Skip alcohol for 24 hours before testing.
- Rest for 10–15 minutes before the blood draw.
Hemoglobin vs Hematocrit: What's the Difference
These two values are closely related. Hemoglobin measures the concentration of the oxygen-carrying protein. Hematocrit (HCT) measures the percentage of blood volume made up of red blood cells. Both fall with anemia and rise with dehydration. If hemoglobin is normal but hematocrit is low, the doctor will look at red blood cell size (MCV) for clues.
When to See a Doctor
- Hemoglobin below 110 g/L in women or below 120 g/L in men.
- Hemoglobin above 170 g/L in men or above 160 g/L in women.
- Symptoms of anemia are present and affecting daily life.
- Abnormal values persist on a repeat test.
The doctor will likely order additional tests — ferritin, serum iron, vitamin B12 — to identify the specific cause and guide treatment.