Hi Liz and welcome! I'm hypothyroid too and yes, it can make weight loss a little more challenging, but certainly not impossible (as you can see from my statistics).
Spoz is right; the key is being on the proper dose of thyroid hormone. I know it can be frustrating to get the dosage right where it should be (I'm in that test every six weeks, tweak, test again cycle right now myself
) but if you're patient and work with your doctor, you can find just the right dosage that will let you lose the weight.
As for diet, a lot of us who are hypo find that controlling carbs is very helpful. I avoid sugar and refined carbs and focus instead on fruits, veggies, and limited amounts of whole grains. You might want to take a look at what you're eating and see if you're a little heavy on the carbs.
And exercise is a necessity for us! Can you tell us a bit about what you're doing now regarding an eating plan and exercise and perhaps we can offer some suggestions?
Don't give up hope! We have quite a few hypothyroid members here at 3FC who have successfully lost weight (many over 100 pounds) and kept it off, so stick around and we'll help you through.