Sorry to hear about Buddy
I would listen to your vets reasons and ask why should that thyroid removed. I know they have two but I think it depends what is wrong with it.
The only knowledge I have is that Kocka was like this and my vet said she had a tumour on one thyroid and it was not malignant but advised the removal of the thyrois cos he knew in her case there was a problem medicating her......I dont know if he would have said this option anyway and it is going back about 8 years ago.
She had the op and before that her kidneys were fine, she was about 15/16 yrs old but tests after the op showed that her kidneys were a problem so have to think the op caused that and suspect that she was not put on a drip......I didnt know any better in thise days and a drip is essentail for all senior cats having an op.
So my thought would be there would have to be a very good reason for surgery...............Oh yes and Kocka never put back on all the weight she had lost and had probs feeding her for the rest of her life..............cos not sure that all Hyper-T cats need an op , just meds.
I am sure others will have more upto date info