How long should I stop PTU before thyroid uptake and scan test

[YoungWood]

I got hyperthyroidism and have been taking PTU for 4 years. I move in California now and see the doctor. The Endocrinologist told me I got grave's and suggested a radioiodine therapy. Before that I need a thyroid uptake and scan test. The doctor repeatedly told me I need to stop using PTU for 4 weeks before the scan test. However, when I make an appointment with the scan test lab, the guy told me I just need to stop using PTU for 3 days. I am confused and called back to the Doc. The doc speaked in a halting manner when I asked why there is so big difference. The doc suggested I follow the lab guy's protocol. Now, I am worrying about the doc and do not trust him. He is a expert and should not made such a mistake.
SO, is it resonable for me to worry about it?

Thank you and I am waiting for your response.

[Mark Lupo, M.D.]

3-5 days off PTU is adequate for I-123 scan and for I-131 treatment.  The reason for the scan is to determine if this is graves or a toxic nodule and to estimate the required dose of I-131 for treatment.

[YoungWood]

Dear Mark Lupo, M.D,
Thank you so much. Your comment is convictive and important for me.