Re: Weight gaining / anxiety ?
I still have my thyroid, so I can't address the anxiety part of your question. There are plenty of members here who've had theirs removed, so I hope they'll help you out with that part of it. I can tell you, though, that some anxiety is closely tied to thyroid function.
I would say the only reason to fear large weight gain after thyroidectomy is a doctor who refuses to supplement thyroid hormone soon enough after surgery or who refuses to prescribe it in a large enough dose to get your T4/T3 levels optimal for you - Not just "in range", but truly "optimal". You have to have a thoroughly competent thyroid clinician. I'll be honest - Good ones are not easy to find.
If you have time and energy to interview some before the surgery, that would be best.
You do NOT want one who treats according only to the TSH level; he must look at your free thyroid hormone levels as well.
He must be willing to consider both your symptoms and your lab results. No MD should treat only the lab sheet and ignore "you".
You do NOT want one who has a favorite drug and prescribes only that one. Real patients don't all respond the same to any one drug. If the MD won't consider switching to something else, the patient will never be well.
Wishing you well, and good luck with surgery.