I think it is wonderful you had 17 years to enjoy and love you dog...it is time to say goodby. We have "had" to put down our last two dogs. Both could have lived a little longer but the quality of life for them and us would have been very low and sad.
It would be Cushing's disease, and your baby will need medication. Think about quality of life at 17 years old. Do you want to put her through it? It usually happens when dogs are older, and that price is a very reasonable one for treatment for a month. Good luck, Prayin for ya.