In general, euthanasia is recommended when a horse is experiencing significant pain or discomfort, or when the disease has progressed to the point where the horse is unable to perform basic functions such as eating, drinking, or moving around. Other factors that may influence the decision to euthanize include laminitis (a serious condition that affects the hooves) which is a common complication of Cushing's syndrome, recurring bouts of colic or abdominal discomfort, muscle weakness or inability to rise and difficulty breathing or chronic lung infections.
The decision to euthanize a horse is always a difficult one, but it is important to remember that the goal is to prevent unnecessary suffering. If you are concerned about your horse's quality of life, please talk to your veterinarian about whether or not euthanasia is the right choice.