Low if careful. Synthroid (thyroxine) is used for under active thyroids. It is usually started at a low dose and levels checked 4 to 6 wks after changes made. Too much thyroid medicine can cause aryhthmias and heart failure. But if thyroid disease is not treated pts can also have arythmias and heart failure. So not taking the medicine is not a good option. If your not sure you need the medicine then get a second opinion.