I am also hypothyroid and gained more than 30 pounds when my thyroid started to shut down. You can see my success with low carbing by looking at my stats. My doc told me that people with low (or in your case no) thyoid do not process carbs normally. They turn directly into fat. The only thing that has worked for me since my thryoid issues has been low carbing. I have had to do Atkins because it is stricter-and my doctor warned me I probably would never go above 40-50 g of carbs a day, even in maintenance.

You might need to get a referral to an endocrinologist. That is their specialty. Check out this site:
http://thyroid.about.com/mbody.htm

Lots of good information here. One thing that you can read about is that normal TSH levels have been redefined. They have been lowered-so that the level of medication you are on now might need to be upped. I had my doc up mine because I wanted my TSH between 1 and 2.