Fatigue is one of the most common symptoms of iron deficiency, Dr. Pathak says. People are used to being in a hurry all the time so many don’t pay attention when their body feels tired. Lack of iron leads lower counts of hemoglobin, which is the protein that helps red blood cells deliver oxygen throughout your body. This means less oxygen is going to your muscles. As a result, they have no energy, leaving you feeling exhausted.