Re: What should I expect?
Ok...there should be some little letters after that ACTH post #...can you let us know what those are? And the range given from the lab that follows that post #? Should be two numbers something like -
210 - 550 (EXAMPLE)
What dose of Vetoryl is the vet recommending? The new starting dose for this drug is 1mg per lb (or 2mg per kg). HOWEVER not all vets are aware of this change and continue to start dogs at much higher dose (3x). So let us know that dose before before he starts taking it. 
It is important that Seymour get the med WITH a meal every time as Vetoryl is a fat soluble drug and must have food to work. Seymour will be very glad to hear this part!
As for diet, it is best not to change anything right now. Changing foods can cause digestive upsets and those very things are also signs that the cortisol has gone too low. So we do not want to muddy the waters right now. Just let things remain the same UNLESS there is a medical reason to change the diet like IBS, allergies, etc.
I hope the vet told you the signs to look for that would indicate the cortisol has dropped too low - a life threatening situation. Just in case those signs are loss of appetite, lethargy, loose stools/diarrhea, nausea/vomiting. If you see any of these things STOP giving the Vetoryl and let your vet know what is going on. And talk with us.
We will help you thru. The good thing about Vetoryl is that is has a very short life in the body so IF there is a problem the drug is leaving the body pretty quickly. It is also easy to stop and restart for that same reason.
You're gonna do just fine, Mom! Keep breathing, ok? You'll be a pro in no time!
Hugs,
Leslie and the gang
"May you know that absence is full of tender presence and that nothing is ever lost or forgotten." John O'Donahue, "Eternal Echoes"
Death is not a changing of worlds as most imagine, as much as the walls of this world infinitely expanding.