About Hemoglobin
Hemoglobin is the iron-containing protein inside red blood cells that carries oxygen from your lungs to every organ and tissue in your body. Low hemoglobin — the defining feature of anemia — causes fatigue, breathlessness, exercise intolerance, and brain fog. The cause of low hemoglobin (iron deficiency, B12 deficiency, chronic disease) determines the correct treatment, so context matters as much as the number itself.