Hmmm....unfortunately I have no idea. My first reaction was thyroid also, but since that has been ruled out.....

Did they do regular blood testing? I know they tested for the thyroid, but what about all the other levels? That might help point them in a certain direction at least.

Unfortunately, since the money is not there for extensive testing, it probably wouldn't be there for expensive treatment either, and she might just have to face that fact that Darling's long life may be coming to an end. There is nothing wrong with that - 17 is a good long life for a cat. Tell her to just make Darling as comfortable as possible, cherish every moment with him, and if he wants a can of food an hour - give it to him.