Ditto what Loosie said but I might take that a few paragraphs further:
I know that, once the horse is started on Thryo-L (L-Thyroxine), it is NOT a permanent thing. It's use on IR or EMS horses is to get them to lose weight when absolutely nothing else has worked.
Also, did the vet tell you the horse has to be backed of Thyro-L gradually? The drug cannot
be stopped "cold turkey". Just like when Prednisone is given to dogs with certain types of allergies, the Prednisone dose is gradually reduced until the dog is completely off it.
A good friend had a Paso Fino with Cushings and Insulin Resistance. She battled the weight issue with him for three years -- dry lot, taping the grazing hole shut so he couldn't eat the tree leaves -- just everything.
She finally had to put the horse on Thryo-L and it did the trick. I THINK the horse was only on Thyro-L for 2 - 3 months; it was a short time.
My point to all that is Thryo-L is not meant to be given to metabolic horses long-term and where IR and EMS are concerned, it is the last resort
to get the horse to lose weight.
The vet is absolutely correct on NO GRAIN. Don't even feed a ration balancer with soy in it. If this is your only horse, you may have to lay out a gawrsh-awful amount of money and buy EquiPride ($60/50 lb bag
EquiPride is a complete vit/min supplement with a pre-probiotic that is soy-free. Horse equine ? EquiPride and EquiLix
Don't buy the lick (well you can't because you board) but anyway, buying the lick means you can't control what the horse eats. The daily feeding amount is only ten ounces.
You could buy straight timothy pellets and mix a dry pound of timothy pellets with the EquiPride. It is a taste that some horse like and others don't care much for.
Tractor Supply carries Standlees timothy pellets; they come in a nuclear lime green bag - lol Make sure you don't come home with the alfalfa pellets.
I suggest this approach because I have two metabolic horses; one with insulin resistance and the other with Equine Metabolic Syndrome.
Oddly enough the EMS caused that horse to drop weight; he went from an air fern to a hard keeper in less than six weeks, once the EMS exploded. That was five years ago.
Unless the BO is willing to bend over backward and do handstands, you're in a really tough situation with your horse. It absolutely needs special care and there's no margin for error<---meaning nobody should be slipping it horse treats because they feel sorry for it.
Your horse is foot-sore right now because Fall grass is every bit, or more, dangerous than Spring grass. My EMS horse amazingly has never even had a laminitis attack and does not wear a grazing muzzle; He gets blood drawn twice yearly. My EMS horse is on chastetree and it's working however, chastetree does not work on the IR horse.
The IR horse is the air fern and his insulin spikes just looking at a blade of grass. He foundered really bad last March because I didn't get the muzzle on him in time. He is STILL going out to pasture wearing a muzzle and will continue to wear one until the daytime temps are cold enough to freeze his nose hairs to the muzzle
My IR horse is on Chia Seeds. They work for him but the horse needs closely monitored when on chia seeds; I noticed this horse went from being a slathery sweat hog in the summer to not sweating at all. I cut the chia seeds waaay back and he started sweating again. He has lost enough weight that I can now feel his ribs if I press. Trouble with the Chia seeds is he doesn't like the taste of them so I only give him 1/8th cup; that's ok with winter coming but I'm going to have to feed him three times a day once Spring gets here.
Then there's the possibility of ulcers developing but I think I've already over-whelmed you.
Just an FYI that Arabs are right at the top of the "Predisposed List" for metabolic issues.
Oddly my 26+ Arab is perfectly fine in that regard. It's my TWH's (also on that Predisposed List:(
I'll stop there but the bottom line is:
1. NO GRAIN whatsoever.
2. Try to get her on a soy-free product like EquiPride.
McCauley's M-10 Balancer is also soy-free. I have tried this on one horse and he eats it without issue. McCauley's« M10 Balancer
3. Grazing muzzle for pasture turnout. If she goes in a drylot with a free access to a roundbale that needs to be stopped. She needs hay but it needs to be weighed.
3.1 Buy a slow-feed hay net for her stall, so she can pick all night as opposed to chugging everything down at once.
4. Nothing but quality weed-free grass hay; NO legume hays such as alfalfa.
5. WHITE salt, no trace mineral salts.
This is more than "a few paragraphs further" and I didn't address the founder issues. You're dealing with a very serious issue that will never go away. IR and EMS are the same as Type II Diabetes in humans; they can be controlled but will always be there