17 1/2 is old for a cat, but my family had 3 cats who lived to be 19. Is there a treatment available for his thyroid? I found this which discusses options. Don't know what you have tried or if your vet thinks any treatment is suitable for him.
If there is not an option to improve his health, I agree with putting him down; it is kinder than letting him suffer.
Sending you some love. I've been there...twice this summer. It's such a tough decision to make. I wish they could talk...
Have you spoken with your vet about options? There are treatments for thyroid issues. The age of your kitty and his general health will be factors. One of my cats lived to be 21 and two others were more than 17. I've been lucky to have a vet who will give me honest answers. I always ask what he would do if it were his pet. Plus I go with my gut. And as terrifying and sad as it's been, when the moment came to make the decision to euthanize two of our cats two months apart this summer, I felt it deep deep inside that it was the best thing to do. I still tear up about it... But, cats live very much in the moment. Quality of life is such an important and sometimes hard to face factor. My vet once asked - Are you keeping him alive for you or for him?
First, find out your options. There are medicines and herbal things you can do for thyroid issues. Call your vet. Search the internet. Don't give up on your kitty...yet don't feel guilty if you come to the decision to ease his suffering.