Having a total thyroidectomy. history hypo, multiple nodules, on synthroid (thyroxine) 100mcg. had horrible wt gain over yr. will i likely gain lots of wt after?

No: You do not need a thyroid, you just need thyroid hormone in the correct dose to maintain a normal TSH level. If your thyroid level remains normal, it should not affect your weight. I assume that you are having a total thyroidectomy because you had a thyroid biopsy that was suspicious for thyroid cancer. If this is not the case, you should get a second opinion from an endocrinologist.