What happens if you decide to try not taking Levoxyl

I have been on Levoxyl or Levoxithrine for almost 6 years now..never had thyroid problem until after I had my son at age 40....they diagnosed me with hypothyroid....I started at .25mcg and am up to 100mcg not....I have gained 20 lbs easy in last few years that I cannot drop at all....I have no motivation and are often sluggish and freezing...I also for the last year and a half have this strange itchy rash that lasts for months...they may appear to look like bites at first and than take on more of an oval appearance and than seem to mesh together...it is very strange as wherever I get them, for instance I will wake up with an itchy foot around 4 in the morning and sure enough when I wake up I have the same rash on both feet...the next morning I may have it behind my knees and the insides of both elbows..now I have it on my scalp and behing both ears and both feet....I went to allergist, dermatologist and regular doctor...they tell me that many people with problem thyroids develope a weak immune system where your body actually attacks itself....does anyone have any idea what this rash could be.......I have also developed an allergy to iodine and shellfish in the last 2 years....does this thyroid med contain anything that would make me allergic also