Historically, the veterinary profession considered fat, cresty-necked horses to be hypothyroid. Research in the last 10 years has shown that hypothyroidism is in fact very rare and usually not associated with these signs in the horse.

Insulin resistance is a reduced sensitivity to insulin that makes it more difficult for the body to transport glucose out of the bloodstream and store it as glycogen. Insulin resistance is very common in horses, and has been given the name Equine Metabolic Syndrome (EMS).

Attention has been paid to the condition primarily because horses that have the characteristics described (EMS) also get laminitis, a life-threatening, crippling condition. There are genetic and environmental factors that contribute to the development of the insulin resistant state. As with people that are insulin resistant, there is a wide range of signs from very mild to very serious.

The classic insulin resistant (EMS) horse is a very "easy keeper" being overweight even when fed very little. They often do have low grade laminitis smoldering along, so are often described as being mildly stiff. Classic EMS horses have a typical fat distribution, with hard, thick fat laid down on the crest of the neck and over the top-line and around the tail head.

This syndrome in horses is different than simple obesity from overfeeding and under-exercise. These horses have very high insulin levels in the face of high blood sugar levels detectable on laboratory tests. EMS most commonly occurs in the pony breeds, Morgans, Saddlebreds, Peruvian Pasos, Paso Finos, Arabians, Tennessee Walkers, Missouri Foxtrotters, and horses of similar type to these.

Horse owners should be aware of this state and how it contributes to the development of laminitis. Early intervention in the form of management changes might save the development of laminitis, a painful, debilitating and potentially fatal problem.