What causes polycethemia?

Polycythemia is primarily caused by an increase in red blood cell production, which can be due to various factors. The most common type, polycythemia vera, is a genetic disorder resulting from mutations in the JAK2 gene that lead to overproduction of red blood cells. Secondary causes may include chronic hypoxia (from conditions like COPD or sleep apnea), tumors producing erythropoietin, or other factors that stimulate red blood cell production. Dehydration can also lead to a relative increase in red blood cell concentration, though it is not a true increase in red blood cell mass.