Yes, it's a malfunction of the pituitary gland.
You need to get your horse off any and all grains and no treats, including NO apples or carrots as they're full of sugar.
The strict diet for these types of horses may actually have some benefit for the navicular, as well
Please go to this website and read as you have time. It's a credible place to get good information. Equine Cushing's and Insulin Resistance Information
Dr. Kellon also has a Yahoo Equine Cushings Group that is free to join. EquineCushings : Equine Cushings and Insulin Resistance
You can also join, TheHorse.com for free and search their archives for credible information on cushings
The vet probably wants to put your horse on either Prascend (a newly approved drug) or Pergolide.
Keeping the hooves properly trimmed, frequently, is crucial with cushings disease, as it is with navicular.
However fast your horse's hooves grow, horses with metabolic issues seem to grow hoof faster, so you might want to monitor hoof growth a closer than what you're used to.
Wall separation can be an issues. Tendency toward thrush can now be issue. Scratches and rainrot might become issues.
Not WILL be an issue, but CAN, as the horse's immune system is compromised.
Add some extra Vitamin E to whatever high quality ration balancer or vitamin/mineral supplement you are feeding.
Be prepared to clip more than once during the warm months.
It is constant monitoring and constant work to manage these horses but, they can have a productive and happy life with the proper care.
Which that care is not written in black and white
What works today may not work next week, next month, or next year.
I have two metabolic horses, neither have cushings, I've changed diets and herbs so many times since 2007 that I've lost count.