An elevated hemoglobin may be caused by dehydration, hypoxia, or polycythemia vera.
Hypoxia may result from high altitudes, chronic obstructive lung diseases (chronic bronchitis and emphysema, when the airways have become narrowed manifesting with shortness of breath), and congestive heart failure.
Polycythemia vera is a blood disorder in which the bone marrow makes too many red blood cells.
People with PV are prone to develop blood clots and often experience headaches, lack of concentration, and fatigue as a common symptoms.

Best wishes!
Marija