Low hemoglobin usually indicates anemia and therefore reduced oxygen-carrying capacity.
Common causes include iron deficiency, vitamin B12 or folate deficiency, chronic kidney disease (low EPO), chronic inflammatory disease, bone marrow suppression, blood loss, and hemolysis.
In endurance athletes, low hemoglobin can also reflect dilutional effects from plasma volume expansion rather than true red-cell deficit.
High hemoglobin suggests erythrocytosis or hemoconcentration.
Common causes include dehydration, smoking, chronic hypoxia (sleep apnea, lung disease, high altitude exposure), testosterone or anabolic androgen use, and primary marrow disorders such as polycythemia vera.
Persistent elevations warrant context with hematocrit, RBC count, oxygen status, and medication/supplement review.
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