I had a TT and RAI in 2014. You are correct in that you will need to be hypothyroid prior to your RAI. I was transitioned onto liothyronine (t3) as it has a shorter half life and this was then stopped for 2 weeks prior to the iodine. I have a friend in Manchester who had to come off levothyroxine for 6 weeks prior to RAI. there may be other regimes so be guided by your hospital.
To be truthful the worst part was being hypothyroid, I became so slow unable to remember anything and would stop sentances half way through unsure what I was speaking about. I was unsafe to drive in the 2nd week. I also was extremely fatigued, but the foggy thinking was scary. I also had to follow a low iodine diet for 2 weeks which was difficult with foggy thinking and fatigue. ( I had to repeat this 9 months later for a scan, but precooked and froze meals to eat during that time)
Having the RAI was not as bad as I feared. You are socially isolated in a room until your radioactivity has fallen to a safer level. It is important to keep your fluid intake up to minimise risk to your kidneys and to suck sweets to promote saliva as the salivary glands can be damaged. The radioactivity is excreted in all body fluids so daily shower and clean clothes. I was able to come home after 48 hours but had to sleep in a separate room from my husband for14 days. You have to go for several gamma scans after the RAI so they can see if there is any spread of the cancer.
I was started back on liothyronine 24 hours after the RAI and then transitioned back onto levothyroxine a fortnight later. I felt tired after the iodine and due to my work mixing with the public remained off sick. I had very dry eyes which took about 6 weeks to improve and altered taste which resolved after 6 months ( sadly red wine tasted foul!).
I hope this is helpful. I wish you well at this difficult time. Happy to answer other questions if you have them.