Due to the thyrogen shortage - i was asked to do the scan by stopping the medication. how many days before the scan i need to stop the meds?

Depends: If you are on thyroxine switch to liothyronine for a month, than go off for 10-14 days. Your endocrinologist will know how to do this.

4 weeks: The objective is to get you into a hypothyroid state where your endogenous thyroid stimulating hormone (TSH) will be the highest resulting in higher chances of finding remnant thyroid tissue/disease. This can be achieved by the administration of recombinant human thyrotropin (thyrogen) or by stopping your levo-thyroxine medication for usually 4 weeks.

Varies: There is a blod test that will determine your tsh. Tsh level increases over time the longer you are off your medications.

Hormone withdrawal..: Usually T4 is stopped for at least 4 weeks. T3 (liothyronine) is stopped for at least 2 weeks. Tsh level can be measured in the blood to confirm adequate withdrawal.