A high hemoglobin level (polycythemia) in tuberculosis of the spine (Pott's disease) is primarily due to chronic hypoxia caused by vertebral destruction and impaired respiratory mechanics. The body's compensatory response to prolonged low oxygen levels is to increase erythropoietin production, stimulating red blood cell production and raising hemoglobin. This adaptive mechanism distinguishes Pott's disease from other options where anemia or low hemoglobin is expected. Therefore, polycythemia in this condition reflects the body's effort to improve oxygen delivery despite compromised pulmonary function from spinal deformity.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.