I have a 24 year old QH/Welsh cross with mild Cushings (on 1/2 a Prascend a day, normal hair coat and sheds on schedule). He lives on field board with a run-in and free choice hay and we hack out about 3-4 days a week. He got through the winter in excellent weight (not fat, just right) with a low trace clip and a medium weight blanket. This spring is the first year that he’s gotten a little cresty and put on a layer of fat seemingly overnight when the grass came in. He’s now wearing a grazing muzzle during the day, I’ve increased his work load, and he’s trimming back down again. But the thing I’m wondering about is blanketing. The weather here in the northeast has been all over the place and we went from lows in the 60s and highs around 80 last week to highs in the 50s and lows in the low 30s this week. He is basically fully shed out at this point and most of his pasture mates are wearing sheets on the cold nights, but I have been leaving him naked to burn calories. I know that Cushings horses can have trouble with temperature regulation, but I have not seen him struggle with cold temps, only heat/humidity. Is leaving him unblanketed actually helpful in terms of burning calories/metabolism or do you think I’m just making my horse be cold for no reason?
I doubt very much that your horse is cold without a blanket when the temp only goes to the low 30s. If he’s not wet and shivering he’s fine. Horses generate heat by digesting hay, so colder temperatures may mean more calories burned. On the other hand, he may eat more when it’s cold to make up for the extra calories burned.
Keep a close eye on his weight if he’s without a muzzle and not stalled at night. Some horses (mine, for example) eat more when they’re not muzzled to make up for what they missed wearing a muzzle. That’s why I stall mine at night.
Mammals do burn more calories in the cold, but also, he’s cold. Keep in mind that if you clipped him before and he’s shedding out on schedule, he’s got less hair coat to work for him, even if he does have some extra pudge. He’s slimming down because you’re reducing his caloric intake, and that is the more effective way for him to maintain condition. If you would have put a blanket on him a month ago, put a blanket on him now.
He’ll tell you if he’s uncomfortably cold. 
Both of my horses had Cushing’s and neither one ever indicated they were cold, even with a week of single-digits or below 0 at night, as long as they were dry and could get out of the wind.
It sounds like your fella has a wonderful life and good on you for giving him 24/7 turnout.
Both my Cushing’s ponies do get cold easy. I keep a sheet on them if it’s in the low 40’s or a blanket when it hits the 30’s. If not, they do shiver.
Even when he is shed out and it drops to the 30’s, it shouldn’t be too cold as long as he can get out of the wet and wind?
If he is shivering or hunched up/ tucked up in appearance and obviously cold then add an appropriate blanket or sheet.
He has a good run-in that provides excellent shelter and hay nets that are always full. I don’t think he is dangerously cold or even uncomfortable. I rode him the other morning when it was in the low 40s and he was fresh and happy. I was mostly just wondering if leaving him unblanketed was actually doing anything positive for him. There are those who think that horses are designed to go unblanketed all winter and come into spring a little thrifty so that they put on needed weight with the spring grass, not excess weight. They claim winter weight loss is a “metabolic reset.” However, I have not seen any kind of research backing these claims and the people making them are not vets, more like folks who don’t vaccinate or deworm and use herbs instead.
Blanketing is tedious, stinky, heavy, and leads to tremendous angst about whether we should have put on the medium-medium or the heavy-medium, or the plain sheet or the slightly-colder-outside sheet, or the burgundy HW with blue piping or the crimson HW with black piping… If your horse can be comfy and happy without a blanket, so much easier!!
Going unblanketed does have a negative impact from the point of view that sometimes it is fun/therapeutic for the human to buy horse blankies. And it is nice to snuggle in between horse and blanket when the human is cold.
But from the horse’s perspective, I think significant or frequent fluctuations in weight (ie,
“re-sets”) are not as healthy as maintaining a fairly stable weight. If OP wants to be “Influenced,” I will offer that His-Most-Rotundness did noticeably well with Metabarol, slimmed right down and looked muscle-y. I put him on it in the warm season and took him off in the winter.
There is a great podcast on this topic by Equestrian Voices called “Why a Hot Bran Mash Doesn’t Keep Your Horse Warm, and Other Blanketing Myths Busted with Dr. DeBoer.” Dr. DeBoer has a PhD in animal science and has done research on this topic, which she discusses. It was was a good listen, and she does discuss why someone with an overweight horse may not want to blanket or blanket less.
This is just the type of information I was looking for! Thank you so much!
I have two senior horses with Cushings (28 and 30 yrs). I blanket them a bit more than I did when they were in their prime. One of my horses shows signs of stress when he is chilly, and the risk of colic caused by that stress is a bigger concern to me than his weight. So if it’s low 30s at night I put a rain sheet on as a wind breaker and he is happy. My other Cushings horse has a history of laminitis and I have found that keeping her from getting cold reduces bouts of laminitis.
I think if your horse isn’t showing signs of stress, and doesn’t have other health challenges, he’s probably fine without a blanket. I have been told that older horses can have trouble with thermoregulation, but over blanketing can be a problem too. My cold-sensitive horse once colicked and the only possible trigger I could identify was that I had significantly over-blanketed him in a paranoid response to me thinking his colic a few days previously was due to being under-blanketed 
I agree, I lean toward under-blanketing rather than risk over-blanketing. My guy handles cold well but distinctly dislikes being hot. He still thermoregulates at 24 the same way he always has and no colics or laminitis so far [touch wood]. I’m going to tape him today and see how his weight is doing. A boarder sent me a picture of him this morning and he’s looking good.
