Hemoglobin is the iron-containing protein inside red blood cells that picks up oxygen in the lungs and delivers it throughout the body. Making it requires iron, which is why low iron eventually lowers hemoglobin.
When hemoglobin falls below normal, the result is anemia — and the most common cause is too little iron (see iron deficiency anemia). A low hemoglobin on a blood count, alongside a low ferritin, points to iron deficiency.