Low hemoglobin is one of the most common abnormalities found on blood tests. Globally, anemia affects about one in four people. Here's what you need to know about its causes, symptoms, and treatment.
What Is Low Hemoglobin
Hemoglobin is a protein inside red blood cells that carries oxygen from the lungs to every tissue in the body. When levels fall below normal, the condition is called anemia.
Anemia is not a disease in itself — it's a symptom. The underlying cause can range from iron deficiency to chronic illness to vitamin deficiency.
Normal Hemoglobin and When It's Too Low
| Group | Normal (g/L) | Anemia threshold (g/L) |
|---|---|---|
| Women | 120–148 | below 120 |
| Men | 130–160 | below 130 |
| Pregnant women | 110–140 | below 110 |
| Children 6–14 | 115–145 | below 115 |
Anemia severity: mild — 110–119 g/L in women; moderate — 80–109 g/L; severe — below 80 g/L.
Causes of Low Hemoglobin
Iron deficiency anemia is the most common cause worldwide. Iron is a core building block of hemoglobin. It depletes with poor diet, chronic blood loss (heavy periods, peptic ulcer, hemorrhoids), or impaired absorption in the gut (celiac disease, post-GI surgery).
Vitamin B12 and folate deficiency — without these vitamins, red blood cells fail to mature properly, becoming large and dysfunctional (megaloblastic anemia). More common in older adults, vegans, and people with stomach disorders.
Chronic diseases — inflammatory conditions (rheumatoid arthritis, Crohn's disease), chronic kidney disease, and cancer suppress red blood cell production.
Blood loss — acute (trauma, surgery) or chronic hidden GI bleeding that goes unnoticed.
Hemolytic anemia — red blood cells are destroyed faster than they are produced. Can be inherited (sickle cell anemia, thalassemia) or acquired.
Symptoms of Low Hemoglobin
Symptom severity depends on how low hemoglobin has dropped and how quickly it happened. Chronic anemia is often better tolerated than acute anemia because the body has time to adapt.
Common symptoms:
- persistent fatigue and weakness without clear reason
- pale skin, mucous membranes, and inner eyelids
- dizziness, especially when standing up quickly
- shortness of breath with previously easy activities
- rapid or irregular heartbeat at rest or with mild exertion
- difficulty concentrating and headache
With iron deficiency, additionally:
- brittle, spoon-shaped nails
- hair loss
- food cravings for non-food items — chalk, ice, dirt (pica)
- dry mouth and cracks at corners of the lips
What to Do If Hemoglobin Is Low
The most important first step is to find the cause — not start supplements on your own. Your doctor will typically order:
- Ferritin — the main marker of iron stores. It can drop before hemoglobin falls.
- Serum iron and TIBC — assess iron transport in the blood.
- Vitamin B12 and folate — to rule out megaloblastic anemia.
- Reticulocytes — young red blood cells that show how actively the bone marrow is working.
Treatment depends entirely on the cause — iron supplements, vitamins, or treatment of an underlying condition.
How to Prepare for the Test
Hemoglobin is measured as part of a standard CBC:
- Fast for 8–12 hours. Drinking water is fine.
- Avoid intense exercise the day before.
- Skip alcohol for 24 hours before testing.
- If you take iron supplements, ask your doctor whether to pause before the test.
Hemoglobin vs Ferritin: What's the Difference
Hemoglobin reflects the current oxygen-carrying capacity of the blood. Ferritin reflects iron stores in the body's tissues. Ferritin drops first — often weeks before hemoglobin falls. This is why doctors sometimes check ferritin even when hemoglobin is still normal, especially when fatigue is the main complaint.
When to See a Doctor
- Hemoglobin below 110 g/L in women or below 120 g/L in men.
- Symptoms of anemia affecting daily life.
- Hemoglobin declining on repeat tests over time.
- Symptoms persist even with a borderline result.