Most often, a high hemoglobin count happens when the body needs more oxygen. The body may need more oxygen due to:

• Smoking tobacco. This can lessen blood flow and cause less oxygen to reach the heart and lungs.
• Living high above sea level. Oxygen levels in the air are lower at high altitudes.

Less often, a high hemoglobin count happens because:

• The body creates more red blood cells to make up for low oxygen levels due to heart or lung conditions.
• The bone marrow makes too many red blood cells.
• Some medicines or hormones cause the body to make more red blood cells. For example, lab-made versions of the sex hormone testosterone are a treatment for some men with low testosterone. This treatment is called testosterone replacement therapy, and it can cause a high hemoglobin count as a side effect.

If you have a high hemoglobin count without other concerning test results, you likely don't have a serious underlying condition. But some health conditions can cause a high hemoglobin count, including:

• Congenital heart disease in adults — one or more changes in the heart's structure that are present at birth.
• COPD — a long-term lung disease that causes trouble with breathing.
• Dehydration — when the body doesn't have enough water and other fluids to work as it should.
• Emphysema — a lung condition that causes shortness of breath.
• Heart failure — a condition in which the heart doesn't pump blood as well as it should.
• Kidney cancer — cancer that starts in the kidneys.
• Liver cancer — cancer that starts in the liver.
• Polycythemia vera — a type of blood cancer.
• Obstructive sleep apnea — the most common sleep-related breathing condition.

A very high hemoglobin count can lead to symptoms. Talk with your healthcare professional if you have symptoms such as:

• Trouble with vision.
• Dizziness.
• Headache.
• Slurred speech.

Keep in mind that these symptoms can happen for many different reasons. It's more common to find a high hemoglobin count during a routine blood test or a test checking for another condition. Your healthcare professional likely will do more tests to help figure out the cause of the high hemoglobin count.