What are diseases that cause high hgb?

Low oxygen, blood do. Low oxygen levels sometimes can trick the body into making more blood, and can be seen in people with COPD / emphysema and even in smokers. Blood disorders like polycythemia vera can do this also. Some medications can cause the HGB (hemoglobin) levels to rise (erythopoetin, testosterone, etc). Some people just naturally have a slightly high hgb.