Hello! Thanks for putting your query in HCM. I am a Gastroenterologist (DM). There may be two reasons for diarrhea
in this scenario
there is increase in gastrointestinal tract which may lead to diarrhea. For this you have to become euthyroid. Meanwhile take some antisecretory agent like loperamide
2. It may be due to radiation enteritis
for this take syrup sucralfate
I hope I have answered your query and this will help you. If you have any further query I will be happy to answer that too. Remain in touch and get-well soon.