Several years back my mare was very overweight and out of shape. It seemed like she was getting fat on air, and while she lost some of the weight with regular exercise we couldn't quite get her where we wanted. My vet ended up putting her on thyro-L, which she stayed on for several years. He suspected IR but felt the blood test had too high of a rate of inaccuracy to be worth it, and I declined the much more involved/more accurate test. She was on a very small dose (half scoop) and that seemed to level everything out. She also got grass hay 2-3x/day, 4 hours of grass time in the spring/summer/fall (drylot the rest of the time), and soaked beet pulp with a grass balancer.
This was all in Minnesota, where the grass can be very rich and we were feeding good quality grass hay, occasionally with a small percentage of alfalfa. When on round bales she seemed to self regulate fairly well.
We moved down to Texas last fall. She lost a little weight in the move and it's taken time figuring out how to put it back on. I ended up taking her off of the thyro-L about a month ago - no change in energy, and she's started to muscle back up a little. She is on coastal hay, 2x/day, and is now getting the Nutrena low starch formula grain as the place I was getting grain from stopped carrying the grass balancer with any regularity. She is on pasture here as they don't have dry lots where I board, and out for 8 hours/day (stalled the rest of the time).
Now, I bought a muzzle for her for the spring grass, but she really hasn't gained much weight being on the grass all day so I haven't had them put it on for turnout.
I have been thinking about doing the IR test with the karo syrup (per my vet's recommendation), as he said this is considered pretty accurate? She doesn't seem to have any symptoms of IR any longer, but it has taken some time to put this weight back on and this is a horse that up until 6-12 months ago, I would have never imagined I'd be trying to put weight ON her. She is 13 this year and we've been battling fat kid status since she was 4-5.
Basically, I don't want to have her out on grass without a muzzle if we're risking laminitis. She's never had a laminitic episode and I would like to keep it that way!
From what I've read online a lot of places just recommend testing glucose, insulin, and leptin?