Summary
Historically, fat horses were thought to be hypothyroid. Many horses were treated with thyroid hormone (levothyroxine sodium) and the horses lost weight. However, we have learned that low thyroid hormone levels are actually very rare in adult horses. Low thyroid is not the reason that fat horses are fat.
Levothyroxine sodium is a synthetic thyroid hormone that helps cells regulate glucose and insulin. It is is helpful in treating insulin resistance/Equine Metabolic Syndrome (EMS), which is extremely common. EMS is the main reason for the use of thyroid medications in horses.
Fat accumulation worsens insulin resistance. Thyroid supplementation increases metabolic rate and reduces fat deposits. This helps horses with EMS and makes it less likely that they will develop laminitis.
Equine thyroid supplements usually come in a flavorless, gray to tan powder intended to be top-dressed on feed (usually a low-carb concentrated feed). The active ingredient in these supplements is levothyroxine sodium, a synthetically produced medication that is similar to the action of natural thyroid hormone produced by the thyroid gland.
Several brands of levothyroxine sodium are currently FDA-approved for use in horses. It is a prescription drug restricted to use by or on the order of a licensed veterinarian.
YOUR VET'S ROLE
Your vet helps you determine whether thyroid supplementation would be a helpful part of treatment and selects the dose. They help you monitor the horse's response to treatment and adjust the dose accordingly.
YOUR ROLE
Ensure that your horse consumes the recommended dose of the recommended brand, as directed by your vet. There usually is a 12mg scoop provided in the container. Horses usually consume this medication readily. However, if your horse is not consuming the supplement, notify your vet immediately.
Sometimes, the powder settles to the bottom of the feed bucket and is not eaten. In this case, you may slightly moistening the grain with a small amount of water to help the powder adhere better to the grain.