Think about it, if your immune cells are low, you could feel tired, catch colds easily and could have a lack of vitamin C and zinc in your diet, or a build up of heavy metals that block zinc. Thyroid hormone moves to every cell in the body to activate the cellular metabolism. When thyroid is even sub-optimally low, one can feel tired, have hair loss, and lack of motivation. Blood sugar also plays a major role in our energy. Too much sugar in the blood is very toxic and affects many systems. Iron is commonly low, as well as B12 and magnesium, all of which can cause some significant fatigue. You want to know if this is even beginning to happen in your body.