My question is, I know that stress can evelvate their ACTH levels,
Absolutely, it seems mostly about cortisol production (Dr Bruce Nock calls Cushings 'hypercortisolism'). Stress at the time of testing can affect results. Be that mental or physical stress. But it's chronic stress, not short term acute stress, that is really bad news. Perhaps your horse has had a chronic, 'low grade' type thing & it was the stress or physical insult on weak hooves that was the 'final straw' when the colt chased him.
and my horse doesn't really have any other symptoms of cushings. Well he does sort of, but nothing has changed in the last 10 years i've had him. He is 23 yr old saddlebred. About 6 years ago we noticed he had a swelling in his throat
That's a rather contradictory bunch of statements!
So... you noticed at least 6 years ago something wasn't right & there were cushings like symptoms. Haven't consulted my notes, can't recall whether cushings can cause hyperthyroidism, or it's vice versa, but I know they're related, so the tumor could have a bearing on Cushings, be it the chicken or the egg. Cushings, like 'low grade laminitis' & such, is generally something that 'creeps up' gradually & often unnoticed, may go on for many years before 'suddenly' a sign or problem is obvious enough to allow it to be 'clinically diagnosed'.
This vet doesn't believe that the thyroid has anything to do with anything.
Thyroid function has an awful lot to do with... an awful lot. Check out ecirhorse.com too, for some really good info on managing Cushings.
I just am scared to ride him now that he has foundered.
Yes, I wouldn't ride him until his feet have healed enough &/or he is well enough padded to use his feet comfortably & correctly. I wouldn't ride a recently foundered horse in shoes either. What are they for?
Comments on the pic; He has that sort of look about him that he's possibly a long-term IR horse?? He's not at all fat, but is slightly cresty & possibly they're 'fat pads' on his rump, near his tail, & he has a 'dropped' looking back. Could be due to cushings instead/as well, or could be no fat pads in reality & nothing to do with that - could be about his hooves, of which are not helping him. High heels & run forward feet could be the reason for his conformation/posture.