High dose therapy for CA thyroid

SIr,
I underwent total thyroidectomy in year 2006 and mediastional sternotomy in 2008 for removal of residual tissue,I underwent 2 doses of high dose 131 radio iodine therapy 150mci in feb2007 and december2007,eeven though my Tg level remians around 40 after stopping eltroxin for 3 week but low dose scan always -ve,even 2 petscan also -ve,with thyroxin 200 per day reading Tg 5 only,
I am having following question.
1. when low dose/pet scan -ve how Tg level remain 40 or above?
2. from where tg is produce?
3.My nuclear scan doctor advise I should go for impirical dose of 100mci,pl give yr.comment
4. is it correct treatment?
5.he says if -ve after getting dose and scan with -ve  I need not have to worry,we will put on life long eltroxin dose is it correct? every year we will do scan for 2 years
6.is person can take dose more than 1500 in his life?