I have talked many times about the importance of glucose reaching the cells from the bloodstream. Our digestive system and insulin and cortisol all play a role in enabling blood sugar to flow into our cells.
Inside our cells the mitochondria use glucose to manufacture adenosine triphosphate (ATP), which is the chemical energy our cells use. Without enough ATP thyroid hormone will not be able to work. So, regardless of the thyroid hormone type, or the dosage of thyroid hormone, if ATP is low then the person will remain hypothyroid.