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What Causes a Low or High Hemoglobin Level?

Hemoglobin is an iron-containing protein that facilitates oxygen delivery to cells in the body. Your hemoglobin level can be measured with a complete blood count (CBC). According to the American Red Cross, a normal hemoglobin level is 13.5–17.5 grams per deciliter (g/dL) for males and 12–15.5 g/dL for females.1 The standard values can vary between different laboratories.

A CBC blood test is often part of a routine checkup, and it is also commonly ordered as part of the evaluation of medical symptoms, such as fatigue and fever.

High hemoglobin levels and low hemoglobin levels are indications of different medical problems. Usually, these conditions can be corrected with medication or other interventions. This article will discuss when low or high hemoglobin may occur, including symptoms, risk factors, and treatment.

A hemoglobin level above the normal value is considered high. There is a variety of causes.



Your body can make excess hemoglobin in circumstances that lower your ability to get adequate oxygen to your cells. Oxygen is needed for energy and cell survival. The excess hemoglobin can help you increase oxygen delivery to your cells.

You can also have a high hemoglobin level if your body makes too many red blood cells, which may happen to make up for low levels, or it can occur due to disease. The excess red blood cells often contain normal amounts of hemoglobin, which raises your overall hemoglobin count.

Low Hemoglobin Level

A hemoglobin level below the normal value is considered low. Usually, a low hemoglobin level is an indication of anemia.



You can develop a low hemoglobin level if you don’t make enough red blood cells or if you lose red blood cells faster than your body can replenish them.

You can also have a low hemoglobin measurement if your blood contains excess fluid, which can occur with some medical conditions, especially kidney failure.


What Is Anemia?

Anemia is a low red blood cell count or diminished red blood cell function. There are many different causes of anemia.


Often, high or low hemoglobin levels develop over time due to chronic disease, leading to effects that can worsen over the course of weeks or months. Acute conditions can rapidly lead to low hemoglobin, which may cause symptoms to develop quickly over days or hours. 

Generally, a high hemoglobin level does not cause symptoms, but it can lead to complications, including blood clots, heart attack, and stroke.2 

A low hemoglobin count commonly causes noticeable nonspecific symptoms. Symptoms of low levels include: 

  • Low energy 
  • Sleepiness
  • Pale skin 
  • Headaches 
  • Dizziness

Very low hemoglobin can cause tachycardia (rapid heart rate), hypotension (low blood pressure), and dyspnea (trouble breathing). 


Risk Factors 

Many conditions can increase the risk of having low or high hemoglobin levels. 


High hemoglobin is caused by:

  • Polycythemia vera (a condition in which there are high levels of all blood cells)
  • Heart or lung disease 
  • Liver or kidney cancer 
  • Chronically low levels of oxygen 
  • Smoking 

Being in a high-altitude location can raise your hemoglobin level temporarily until you get back to a lower elevation or until your body adjusts to the atmospheric pressure at high elevation.


You can have a high hemoglobin level on a blood test if you are dehydrated. This does not reflect a high level of hemoglobin in your body, and the measurement would be normalized once you are adequately hydrated. 


Low hemoglobin is caused by: 

  • Low iron intake in the diet or low iron absorption 
  • Menstrual bleeding 
  • Bleeding from any cause, including surgery,3 trauma, or a gastrointestinal bleed from an ulcer 
  • Cancer, especially blood cell cancer 
  • Chemotherapy

You can develop a low hemoglobin level if your body isn’t making enough red blood cells or enough hemoglobin. Low hemoglobin levels also can result if your red blood cells are lost due to bleeding or become damaged due to disease. 

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