I had Graves disease a few years ago which resulted in me having radioactive iodine and subsequently, no functioning thyroid. Now I have hypothyroidism and I take thyroxine every day.
I had my surgery last December and I've currently lost 47kgs (starting from 147kg), my thyroid levels are completely normal with help of the medication. I get blood tests every 4 months just to keep an eye on it.
So it is possible to lose weight, even with a thyroid condition, occasionally it feels like it's coming off a little slower, but as long as my levels are right, there's no reason I can't lose the weight.
Best of luck for everything with your surgery.