I am so very sorry to hear about your dear Stella. Our cat is also named Stella and we adore her every bit as much as you love your dear lady.
As distressing as it is that her radiation appointment was cancelled, it does sound like you managed to find a medication dosage that Stella could live with, after you removed her awful collar! At least that was what I thought I read; please correct me if I am wrong. But if I am right, hopefully her T4 levels will reach the appropriate level soon.
It might be wise to get a second opinion from a specialty veterinarian if that is something affordable (these specialists can be extremely expensive). I wonder if somehow the medication itself zapped Stella's thyroid and she is now no longer even in need of the radiation therapy? In any case you need and deserve answers.
Reading ahead I see she has put on some weight and seems to be doing well. All you can do is maintain this positive regimen, possibly get a second or even third opinion, and continue monitoring her T4 levels.
If necessary keep scheduling radiation appointments so when her body is ready, assuming she still needs this treatment, she will be treated ASAP.
Years ago I had another kitty who passed away from old age in 2010. In 2004 she had the radiation procedure. We were told that while a tumor does cause the condition, that most of the time they are not malignant. My cat's wasn't. How does your vet know that Stella's tumor is in fact malignant? Have they tested it?
My understanding from way back when was that there was really no way to tell for sure but the odds were very strong that the tumor was benign.
You are a wonderful cat mom and Stella knows it for sure!
