Did the vet suggest doing the ACTH blood test for cushings? If he didn't, you need to ask him to do that before the horse starts "gathering" her winter coat and storing winter fat.
Don't let him do the Dex test (dexamethasone), because it can bring on founder in some horses.
Did the vet suggest a feed plan? I'm not surprised if he/she didn't. While some vets are up-to-speed on cushings and metabolic issues, most are not.
Sooooo get that horse OFF grain five minutes ago. Put a grazing muzzle on her to send her out to pasture, or dry lot her with measured amounts of hay.
By measured, I mean on a scale. She should be fed 1.5% - 2% of her DESIRED body weight, not her current weight.
Some bagged feeds are good for these types horses, most are not. Triple Crown Lite is the best by far, if you have access to Triple Crown.
Otherwise a good quality ration balancer would work.
Cushings affects the pituitary gland, which is why you might see the term PPID used; that stands for Pituitary Pars Intermedia Disfunction.
Pergolide works on these cushings horses but it is not recommended if the horse has metabolic issues such as insulin resistance or equine metabolic syndrome.
A separate blood test to determine insulin level would settle that question.
A horse CAN INDEED, be both cushings and insulin resistant and in that case Pergolide is still needed. My friend's 22 yo Paso Fino is cushings and IR and he is on .5 mg of Pergolide but the dosage needs upped.
I have two horses formally diagnosed as insulin resistant. These are the reasons why I know this much and believe me, I wish I didn't.
It is also critical to know the sugar/starch value in your hay. If you can't store up a lot of hay and buy from different sources, instead of sending it out for testing, just know that you have to cold-soak all the hay this horse eats for 30 minutes minimum forever after.
If the horse does start to get the Yak-looking coat, it will have to be body clipped. My friend is up to clipping her Paso twice during the warm months and he grows that hair back faster with each passing year.
1. NO GRAIN
2. Put on a high quality bagged feed for these types horses or a ration balancer. I would prefer a ration balancer that is vit/min fortified.
3. Grazing muzzle for turnout or put in a dry lot with weight-measured hay (that would be dry before you soak it).
3.1 If dry-lotted, scatter the hay around so she has to move because she needs exercise.
This Southern States link talks about feeding insulin resistant horses, but it's the same game plan for a cushings horse. Feeding the Insulin Resistant Horse - Southern States
Cushings/founder is really serious business. I hope your vet was a lot more concerned that what you saying because I would like to take him/her to the Woodshed for a Come-to-Jesus-Meeting, if he isn't.
I don't mean that against anything you said but I am curious just how concerned (or not) the vet seemed to be.