Is Thyroid Supplementation necessary long term for IR and PPID horse #Thyro-L


My horse Buck is a Spotted Saddle horse 18 yrs old.  5 years ago he had his first attack of laminitis and was diagnosed as IR at that time.  He was put on a carb restricted diet as well as limited grazing AND a full dose (4 scoops per day) of Thyro-L.  Buck did have a cresty neck and a fatty area on his croup but he was not overweight.   He recovered from the laminitis and was doing well until a second episode approximately 2 years later.  As part of that recovery he was allowed limited grazing only with a grazing muzzle and his Thyro-L dosage upped to 6 scoops per day.  He had no other laminitis episodes until last fall (2021) when he had 2 episodes back to back.  He was then diagnosed with PPID and in Sept/Oct timeframe put on 1 Prascend tablet at his evening meal.  He has slowly recovered from these laminitis attacks and seems to be doing well now. My vet recently lowered his Thyro-L dosage to 4 scoops per day.   Although Buck is doing well now, I am concerned about a recurrence of laminitis this fall even though my vet doesn't recommend changing his dosage.

I have read recently that Thyro-L is commonly prescribed for horses when in actuality they do not hypothyroidism.  I believe Buck was initially put on Thyro-L because of his IR diagnosis and we just have never revisited that prescription recently.  My vet is now saying he agrees that we take Buck off Thyro-L but has not told me how to wean him off or if we need to test him periodically to see if his thyroid begins to function normally on its own.  Any guidance you can provide will be greatly appreciated.

Buck has been on the following diet for the past 5 years:
Bermuda grass hay free choice - tested and usually ESC + starch = around 5%
Safe Choice Special Care - 13% starch  - around 1 lb per day
Thyro-L - 4 scoops per day now
Vitamin E (Elevate Maintenance) - 1 scoop per day
Vigortone Triple V Free Choice Minerals for horses - 1/4 cup per day
Iodized Salt - 1 Tbsp per day
Stock Salt - free choice
Fastrack probiotic - 1/4 cup per day
Grazing - Bermuda grass - early morning before 11 am always with a grazing muzzle
Exercise - minimal due to my health issues and not having anyone who can exercise may horses. He and my 2 other horses have a large turnout area 24x7 as well as have limited pasture grazing. I am recovering from recent foot surgery right now and hope to be able to begin exercising Buck again in the next month.

Please let me know if you need any additional information.  And thank you for your help.
PMC in TX 2022

Sherry Morse

Eleanor Kellon, VMD

Sherry did a great job of outlining the issues here. I would just add that the fall timing of his laminitis last year is highly suspicious for uncontrolled PPID during the seasonal rise.

THYRO-L DOES NOT CONTROL INSULIN. Get him off it and forget about it.

What is his current ACTH level? There is very little chance that a horse with this history will be protected by 1 mg of pergolide. If you haven't tested ACTH recently, now is the time to do it.
Eleanor in PA 
EC Owner 2001