Stopping your meds will not make the cancer come back. If the cancer is going to come back, it will do so regardless of if you take the meds or not. The meds are not chemotherapy. They're replacement thyroid hormones that you need since you only have half of a thyroid and it can only make half of the normal amount of thyroid hormones.
Now that being said, you need to take the meds!!! And you need to keep up with your blood work to make sure your thyroid hormone levels are normal. Worst case scenario is that you go into a myxedema coma and die from hypothyroidism cause by not taking the meds.