If you have hypothyroidism, the medical term for underactive thyroid, the treatment is to take thyroid hormone in a dose high enough to lower your level of TSH, thyroid stimulating hormone, to a normal level. Though hypothyroidism can cause weight gain, you can't "blame" your thyroid condition for your weight once the TSH is in the normal range. Of course gastric sleeve is a great operation to help with weight loss in appropriate patients. You may be one of them.