Can low thyroid or hyperthyroidism cause death?

Yes: This used to be fairly common. And perhaps almost as bad, untreated hypothyroidism will slowly erode your life so that you cannot do any of the things you enjoy. Your mind will slow and perhaps go insane, your heart can't keep up, you'll get fat, grotesque-looking and weak and be severely constipated. And eventually you get myxedema coma and die. Treating it is extremely easy if done on time.