If i take medicine for overactive thyroid, will i gain weight?

Not necessarily: You will return to normal physiology once you stop burning off extra calories. Once your thyroid is normalized, it will be like anyone else with a balance of what you take in and what you burn off. Many people, however, learn to eat more when their thyroid is overactive and need to "unlearn" their overeating when the thyroid is normalized. You body won't do it for you.