What are the possible reasons for abnormal levels of Hb?

Possible reasons for low levels of hb:
Malnutrition and nutritional deficiencies of iron, folate, vitamin B12, vitamin B6
Red blood cell destruction due to a transfusion reaction
Haemorrhage (bleeding)
Lead poisoning
Excessive intake of fluids (over hydration – temporary dilution of haemoglobin level)

Possible reasons for higher levels of Hb:
Congenital heart disease
Lung diseases like cor pulmonale and lung fibrosis
Polycythemia vera or increased RBC formation due to excess of erythropoietin.