I had my RAI treatment on the 1st of this month, after a thyroidectomy in March and radical

neck diss in May for thyroid papillary cancer. I go back on the 13th for them to do the scan that shows the uptake. I'm sure I asked my doctor about this and am being forgetful, so surprising since I'm hypo, if the cancer has spread beyond the neck or if there is any remaining in the neck, it will show, correct? If it does show anything other than uptake in the thyroid, does that mean more treatment, or is it possible that this RAI will still kill it off, but it may take awhile?
Also, is lightheadedness and/or dizziness a hypo symptom. Just curious b/c they put me on some other med after the RAI which made me very ill, and I'm trying to distinguish what's coming from what.
Thx,
Lisa