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:

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