Hello and welcome from me, too! Terry has already covered the bases re: diagnostic confirmation. But if it turns out that your dog truly has Cushing's, here are a couple more thoughts that I would add re: proceeding with treatment.
Cushing's is typically a slowly progressive disease but it does have the potential to cause systemic damage over time (from high blood pressure, vulnerability to pancreatitis and infections, high cholesterol, kidney damage, liver inflammation, etc.). So for a younger dog, I'd certainly recommend effective treatment so as to eliminate some of these risks, improve longterm quality of life, and allow the dog to live out his/her normal lifespan. However, for a dog of your girl's age, immediate quality of life issues seem of paramount importance to me. And there are some trade-offs to treatment, especially for an arthritic dog since the arthritis may actually worsen as the cortisol level drops, necessitating the introduction of additional drugs to better manage the discomfort. Also, especially at the beginning, there can be numerous vet visits and blood draws which can be less than pleasant for a dog who's nervous at the vet (and also hard on the owner's pocketbook
).
I don't tell you any of this to dissuade you from treating if you and your vet agree this is the best path forward for your dog. For instance, if there are significant amounts of protein in her urine, uncontrolled Cushing's may indeed be playing a part and lack of treatment may allow kidney problems to progress, perhaps even rapidly. But again, at her age, I do think you are the best judge as to how uncomfortable her current overt symptoms are to you both, and if they are not bothering her all that much -- as I say, you may want to hold off on treating for the time being. You are both so lucky to have shared all these years together, and given her life expectancy, I realize that extra quality time from this point onward is a gift to be savored.
As far as her lack of response to the Vetoryl -- if she does have Cushing's, this may simply have been the result of a dose that wasn't high enough. Starting doses are based on weight, but subsequent adjustments are often needed once monitoring testing is performed. This is because every dog metabolizes the drug differently, and the optimal dose must be tailored on an individual basis.
Just a couple of thoughts to throw into the mix.
Marianne