I read something pretty interesting in Dr. Broda Barnes book,"Hypothyroidism the Unsuspedted Illness", that when he was a young researcher they took some rabbits and removed their thyroids. Well guess what the rabbits died! Makes you decide to take your meds and keep up your health. Another thing that a properly acting thyroid does for you is boost your immune system. This keeps you from picking up every germ that comes along. All this boils down to a long and happy life. Just be sure to get the right doctor and correct dosage of medication. There is more in thyroid diagnosis than the TSH. Also be your own advocate. I went 30 years without knowing why I really needed my thyroid meds. Now that the internet has entered my life it is kind of my mission to pass the word. Lots of luck.
It depends -- if your thyroid is severely underactive (ie, longterm hashi, surgery, or I-131 treatment) then you will eventually go into a coma (usually takes 2-4 months).
So it depends on why you are on thyroid meds -- in general I would not stop a med unless instructed by your doctor -- if unclear still, check with a thyroid expert.