The realtionship between thyroid, IR and Cushings is complex. It can be exhibited many ways and for many reasons…but they are not necessarily a succession or progression into Cushings.
IR can be caused by many things…some believe genetics is the key. That some “easy keeper” or hardy breeds (ponies, Morgans, Arabs, some QHs, some Apps, Pasos, Mustangs…) have a “hardy” gene that allows them to survive during periods of drought, winter, or other famine. This genetic ability produces fat stores for future use and allows them to live on very low carb diets. Therefore, this is not a “disorder” but a way of life for these horses.
Other things related to IR are mineral imbalances: generally magnesium deficency as Mg is key in celluar function and reception, hormonal imbalances: thyroid and pituitary outputs impact most of the body’s hormonal out puts either directly or indirectly. When the pituitary is disrupted with a tumor (Cushings) the body produces high amounts of cortisol which effects the thyroid and other hormones. High cortisol out put can also be found in horses in chronic pain. Thyroid can be disrupted by many things…mineral imbalances (specifically selenium and iodine deficency, possibly magnesium deficeny). Thyroid is also effected by stress, age and pain. As age increses, metabolism and exercise generaally decrese.
The realtionship of IR and thyroid is not neccessarily related, but can be found in some and most likely related to similar mineral imbalances. The relationship between Cusings and thyroid is commonly found as the cortisol effects the thyroid and there is general hormal imbalances throughout the body. The development of the Cushings tumor is probably a mystery as are all similar things. There may be genetic predisposition, it may be related to old age, or it may just be “shit happens”. The million $$ question would be is it “caused” by IR or thyroid?
So far, we and many others doing field trials have found that management through diet is the ultimate key to preventing chronic laminitis and in keeping Cushings horses healthy, managing insulin levels and restoring hormonal balance. There are some who indicate that their Cushings horses are healthy without meds and only dietary controls, but I would have to question the diagnosis as Cushings testings is a relatively new thing and we have found that many Cushings horses are misdiagnosed.
My Cushings horse is on the same “all forage” diet that I provide for all my horses now. He is for some strange reason only Cushings and doesn’t exhibit any other problems like IR, carb intolerance, or thyroid disfunction. However, we caught it very early and provided the dietary changes and put him on Hormonize (Evitex) right away. He improved (was depressed, not eating well, saggy belly, swayed backed, long coat, compromised immune system, gained a ton of weight overnight, cresty neck, fat pads on shoulders/tail head) greatly and now only exhibits the long coat, but that is improving each time he sheds. He did not respond to magnesium supplementation as most horses would (reduction in crest/fat deposits). He has never foundered. I also have an IR Peruvian and a “hormonal” problem mare that we believe is hyperprolacticemia…but have no way of knowing. She has a good dispositon, and looks in all ways healthy. No Cushings, no cresty neck…but her udder and the area in front swell in cycle with the cresty necks I have tracted…so I guess this is why of exhibiting the same thing. She was a chronic founder case that came to us when the owner could not deal with it. She tests within normal ranges for insulin/glucose/thyroid.
The diet that I provide is year old grass hay (or soaked new hay for the IR and hormonal horses), beet pulp ajusted according to how much they need to maintain/gain weight, alfalfa/mix cubes (also ajusted for weight maintenance), black oil sunflower seeds, flax meal, kelp, a vit/min mix, probiotics, additional magnesium and other sups as needed per individual. All of this is no more than 20% of their overall diet by weight. I have found that by adding MSM, that I have eliminated all joint supplements except for the Peruvian who had suspensory issues when he came to us. He gets hylaronic acid in place of glucosamine supplements as it addresses his issues better and glucosamine can raise insulin levels. I eventually arrived at this mix as I had to eliminate all commercial feed products and supplements (such as the one I was using for protein) because the carb content was too high for the IR and hormonal horses. The “normal” horses are also doing great on this diet and look the best ever.
Proud member of the * Hoof Fetish Clique *
It seems to me that a lot of people go into this blind. But it’s such an individual disease that not every treatment works for every horse. So you can make yourself crazy over every little detail of your horse’s lifestyle, and it still might not work. Not that it isn’t worth trying, but I wonder if this isn’t the reason why most vets take a minimalist approach to Cushing’s treatment. Or maybe it’s because we just don’t know enough about it yet. All I know is that I wish all this information had been available to me 4 years ago. 
