Yeah if you have a overactive thyroid, it could worsen during surgery. You might end up getting a thyroid storm, which is a massive increase in your thyroid hormones. Thyroid storm could be a life threatening emergency. So please take antithyroid medicines till you get resolved of it and then proceed with your Sinus surgery.

Thyroid storm means nothing but you have severe hyperthyroid symptoms like sweating anxiety palpitations and even rhythm disturbances in heart which can be dangerous.

Neomercazole might take a few weeks to get effective and in any case is only good as long as you take it. Waiting till march is a good idea. By that time let's hope your thyrid hormones are under normal level.

More permanent remedies are Radio iodine therapy which will take away all your thyroid tissue so that you might require to take thyroid supplements for the rest of your life.