A way to improve or resolve a condition or diagnosis. This might include resolving the underlying cause or treating the signs of disease (symptomatic treatment).

Cost: Under $100

These cost ranges are approximate and may vary from region to region.
Additional charges may also apply.

YOUR VET MAY Recommend

Thyroid Hormone Supplementation, Levothyroxine Sodium

Cost: Under $100

These cost ranges are approximate and may vary from region to region.
Additional charges may also apply.


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 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.


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.

my vet's role


Once therapy is started, only discontinue the use of levothyroxine under the guidance of your vet.

Do not discontinue thyroid supplementation suddenly, especially for horses on high doses, and for those particularly at risk for laminitis. Taper to 1/2 the amount daily for 2 weeks, and then 1/2 again daily for 2 more weeks before stopping, or as directed by your vet.

The potential side effects of levothyroxine include weight loss, which is seen in horses that are over-supplemented and/or underfed.

High heart rate and respiratory rate are considered signs of hyperthyroidism or oversupplementation with levothyroxine, but in reality these signs are rare in horses treated with this medication.

Do not change brands or use an unapproved brand without veterinary guidance.

In cases of competition horses, always check with your vet or the appropriate governing regulatory body regarding use.


In most cases, there are minimal complications for the use of thyroid hormone supplements in horses.

It is not approved for use in pregnant mares but clinically seems to be safe. Levothyroxine should not be used in horses with certain heart conditions.

Your vet should guide your use of the product.

your role

Is it working? Timeframe for effect.
It may take weeks to months to notice the weight loss that usually results from the use of thyroid supplements.
Questions To Ask Your Vet:
  • Do I need to continue this medication?
  • Am I giving my horse the appropriate amount of thyroid supplement?
  • If my horse is likely not hypothyroid, why do you suggest I use thyroid supplement?

Author: Doug Thal DVM Dipl. ABVP