I'm so sorry you've suffered the loss of a lifetime companion like this. 17 years is remarkable, certainly indicating exceptional care, but also a long time to have someone in your life and a huge emptiness when they're gone.
What you did was the right thing. It was the ultimate act of selflessness and kindness. Once a vet finds this degree of liver failure and pain, a recommendation of euthanasia is the right and often difficult next step.
It sounds like you kept her alive through some very trying episodes leading up to this point and provided the extra time that many cats wouldn't have had.
As for what it might have been to cause this, sometimes it's never known. Even with a necropsy, an 'undetermined' causation isn't unusual. With all of our knowledge and expertise in this world, we just don't know everything and are reminded of our being 'only human' with losses like this.
What matters is that she knew only love and her life with you was a good one.
Now it's a matter of time before you stop hurting so much.
Tumors can be too small to see on X-ray or MRI or even CT. Bloodwork is often not indicative.
I kept reminding myself that we all have to pass from something and it's not so important 'why' the end comes as it is that we had a good journey to that point.
Your little one had a good life.
You can follow up as much as you'd like, don't worry about that ok?
The diabetes could lend to the elevated liver enzymes, but from your description she had more going on than this. I truly feel you did everything you could do, including the kindest end.
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Animal Rescue&Care Org Owner; Animal Care author; Animal Behavior Consults.;Cert. Avian Specialist;