First off -- let's hope it is NOT Cushings. Cushings is not easy to treat and at best you "manage" it -- essentially the body is producing too much of it's own cortisone ("cortisol";) and most of the Cushings drugs essentially disable or kill part of the adrenals to get them to produce LESS Cortisol.
I've had two different dogs with really horrific skin problems (Muffin the Intrepid and Billy) -- and early on, I decided that If I EVER had a dog with Cushings I would treat it homeopathically. At this point I might entertain TCVM (Traditional Chinese Veterinary Medicine).
Why? Because Cushings is a body balance problem and most dogs actually respond really, really well to alternative treatment for this. Before I even "go there" with you, I want to encourage you to read a book: The Nature of Animal Healing by Martin Goldstein, DVM -- I linked that to the Amazon page for the book.
It is probably THE most life-changing book I ever read. Simply because it was the book that introduced me to both homeopathy and "alternative" stuff -- but mostly because he makes so much sense. You will want to zoom to the pages on Cushings.
The other book I'd encourage you to read would be "Four Paws Five Directions" by Cheryl Schwartz, DVM (that's the TCVM primer for the lay person).
There are others that talk about homeopathy and alternative stuff -- but those two are gold. Particularly Goldstein's book.
I want to make a huge point here - I am **NOT** talking about you treating this yourself, nor am I talking about you just following something you read online. I am encouraging you to find a ****GOOD**** vet who does homoepathy (often to the exclusion of all else because if they are GOOD they're known for it and often don't have a whole lot of time for anything else). Or find a vet who does TCVM (again one who does ONLY TCVM or it is the bulk of their practice - not just a vet who dabbles in a little of this and a little of that.
Typically vets like this charge differently than regular vets. There is typically no "testing" (because they have different criteria and the look at the body differently) but the vet will take far more time with the dog -- rather than a 5 - 15 minute vet visit -- it may be more like 90 minutes to 2 hours. so the intial consult is often more expensive. But ... in the long run, you spend far less on tests, and meds but you will need the occasional "tune up" visit to the vet and you will need the remedies or herbs (depending on which you opt for) prepared. In particular, homeopathic remedies are very very individual -- Dog A and Dog B (or Human A or Human B) who take a homeopathic approach to Cushings likely won't have identical remedies.
Aside from the fact that it treats Cushings with literally NO bad side effects, Homeopathy is going to be a lot cheaper. Now there are various different "types" of homeopathic and even TCVM treatment and if you are interested, email me. Essentially the goal is to restore the animal to health, rather than merely "maintaining" the disease.
However, the **first** thing I would suggest -- and suggest EVER so strongly -- is for you to do another thyroid panel. But this time, ask the vet to send it either to Dr. Jean Dodds, directly at Hemopet (http://www.hemopet.org) **OR** send it to Michigan State Vet School.
Why?
Because either of those is going to give you a breed specific thyroid panel. Yes, I realize this dog is likely a mix, BUT the breed specific panel could literally completely change this whole scenario. I did FIVE thyroid panels on Billy back after we got him. He had, without exception, the absolutely WORST skin I have ever seen in my life. We also did at least five, or more, Bile Acids tests (which is one of the most definitive tests for Cushings -- or at least it was 5 years ago) because Billy ALSO was "Cushingoid".
"Cushingoid" is a phrase vets will use when a dog exhibits the classic symptoms of Cushings -- but maybe doesn't actually 'test' to have Cushings. OFTEN there are other factors that actually cause the skin to be so bad. One of those can be low thryoid.
The breed specific test is simply more accurate. The typical thyroid test a vet sends out measures every dog against the same chart. So a beagle is held to the same criteria as an akita ... or a sheltie ... or a St. Bernard or a rottie ... or a bulldog or a chihuahua.
We all know that different dogs have different metabolisms and a LOT of that is because different breeds have different energy levels (which is tied TO metabolism).
When we tested Billy all those times for thyroid it always came back 'within norms' but maybe a little low - but not low enough for them to supplement. However -- when we had his little buffy cocker butt tested at Michigan State? WHOA - IT CAME BACK ****LOW****. We began to treat the thyroid and hallelujah. SUDDENLY something began to work.
Billy's Cushingoid appearance came mostly from the steroids and immune-suppressors he took for IMHA. But even then I didn't just use regular vet medicine -- he also saw a holistic vet who treated him homoepathically **in addition** to the big drugs he took for IMHA. Literally the holistic vet viewed himself as "complimenetary therapy"
Doing both wasn't cheap. I won't lie to you. But IMHA is terminal (and often times so fast you can't even get it treated)
But looking at Cushings in your situation -- it's possible because of the longer term diabetes. (it's all the endocrine system here) -- HOWEVER -- the thyroid could be exacerbathing it far far more than need be. And once you start treating for Cushings you can't really stop, if I recall correctly.
I don't know how open you are to alternative stuff -- but in this case it could be far far better simply because the meds themselves are so much less harsh. That is what prompted my mental decision all those years ago --
The interesting thing is, that my interest in homeopathy has grown -- not just because it's "alternative" but because I've used it to treat my own problems. Altho not on my own -- I have a homeopath I visit who treats the auto-immune problems I have (asthma, psoriasis and rheumatoid arthritis). So I've seen an incredible degree of success. As it happens -- I found the guy *I* go to by asking my homeopathic vet who HE goes to.
Sorry this is so long -- feel free to email me. I know this is a scarey thing -- some people are terrified of being able to "trust" alternative medicine. But the biggest challenge is honestly to find the right practitioner.