Hemoglobin
What it is
The protein inside red blood cells that carries oxygen from your lungs to the rest of your body.
How it can affect you
Low hemoglobin can cause fatigue, shortness of breath, cold hands and feet, and reduced exercise capacity. High values can appear with dehydration, smoking, or living at high altitude.
Men vs. women
Reference ranges are typically higher in men (about 13.5–17.5 g/dL) than in women (about 12.0–15.5 g/dL). Menstruation, pregnancy, and postpartum recovery are common reasons for lower hemoglobin in women.