Iodine is an essential trace mineral required by horses and all mammals. It is used to make thyroid hormones that control metabolism in all cells of the body.
Forages and grains are typically low in iodine because most soils have low concentrations of this mineral. Horses that do not get enough iodine in their diet can develop subtle signs of deficiency including dry skin, dull coat, poor appetite and low stamina.
The classic sign of iodine deficiency is goitre – an enlarged thyroid gland. However, only a few cases of severe iodine deficiency have been reported in horses.
Iodine excess in the diet can also cause thyroid dysfunction and lead to similar symptoms. Horses showing signs of thyroid dysfunction should be evaluated by a veterinarian to assess their iodine status and levels of thyroid hormones.
Most commercial feeds contain iodine in the form of iodized salt, calcium iodate, EDDI or kelp meal. To understand whether your horse is meeting their requirement for this mineral, submit your horse’s diet for evaluation by our equine nutritionists.
Importance of Iodine for Horses
Iodine is an essential mineral for horses and other mammals.
More than 75% of the iodine in the horse’s body is found within the thyroid gland. The thyroid gland is located in the neck near the jawbone and is wrapped around the trachea (windpipe).
Iodine is a component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3).  T4 is released from the thyroid gland into the bloodstream and is converted into the active version (T3) in the liver and kidneys.
The thyroid hormones T4 and T3 are important regulators of:
- Basal metabolic rate
- Heart health and function
- Digestive health and function
- Brain development
- Muscle development
- Skeletal growth and development
Ensuring that your horse gets enough iodine in the diet is critical to supporting thyroid function and producing thyroid hormones that regulate almost every cell of the body.
Iodine in the Equine Diet
The National Research Council’s Nutrient Requirements for Horses (2007) states that horses require 0.35 – 0.4 mg of iodine per kilogram of dry matter consumed per day.
This equates to approximately 3.5 – 4 mg of iodine per day for a 500 kg (1100 lb) horse. 
The amount of iodine found in forages varies by geographic location, proximity to the ocean, plant species and environmental factors. On average, iodine levels in hay are around 110 – 130 ug per kg, equivalent to 0.11 – 0.13 ppm iodine. 
A horse consuming 10 kg (22 lb) of hay with 0.13 ppm iodine would obtain 1.3 mg from the hay. Therefore, hay alone is unlikely to meet your horse’s iodine requirement.
Iodine Sources for your Horse
Horses consuming only hay or pasture will typically require a supplemental source of iodine to meet their requirements. Typical sources of iodine include:
- Iodized salt
- Calcium iodate
- Ethylenediamine dihydriodide (EDDI)
- Kelp meal
Most vitamin and mineral supplements contain added iodine in the form of EDDI or calcium iodate.
If you are feeding a complete feed or ration balancer at the recommended level, your horse is likely getting enough iodine. However, commercial feeds are often fed below their recommended level, resulting in iodine being undersupplied in the diet.
Iodized salt can be used to increase iodine concentration in the diet while also supplying sodium which is an important electrolyte.
To meet your horse’s sodium requirement, we typically recommend feeding 1 – 2 tbsp (15 – 30 grams) of salt per day. Using iodized salt will provide 0.67 – 1.34 mg of iodine per day.
Kelp meal has high iodine content and has additional benefits for supporting gut health. It acts as a prebiotic to support hindgut function and can help reduce the incidence and severity of gastric ulcers. 
Iodine Deficiency and Toxicity in Horses
Iodine is required to make thyroid hormones which affect a broad range of functions in the body. Consuming too little or too much iodine can both lead to thyroid dysfunction and abnormal levels of thyroid hormones in the blood.
The classical sign of thyroid dysfunction is an enlarged thyroid gland, known as goitre. Both iodine deficiency and toxicity can result in goitre. It is therefore not a reliable indicator to distinguish iodine status.
When iodine levels are low in the body, thyroid hormone production will be impaired (hypothyroidism). The thyroid gland enlarges to try to capture more iodine to be able to make sufficient thyroid hormones.
Paradoxically, goitre and hypothyroidism also occur when iodine levels are too high. This is because iodine can directly inhibit the synthesis and secretion of hormones from the thyroid gland.
Iodine deficiency (hypothyroidism) is poorly documented in horses. However, in case reports of adult horses with known hypothyroidism, the predominant signs include: 
- Rough coat
- Dry skin
- Muscular weakness
- Poor fertility
Over-supplementation of iodine in any form can lead to toxicity symptoms. According to the NRC, the maximum tolerable level of iodine for a 500 kg horse is 50 mg per day.
Signs of iodine toxicity include: 
- Poor and abnormal coat
- Skeletal abnormalities
- Poor appetite
- Thickened/abnormal skin
- Low core body temperature
Care should be taken when estimating iodine status in your horse as the symptoms are vague and easily confused.
If you suspect your horse has thyroid dysfunction, consult with your veterinarian for a diagnosis. Blood levels of thyroid hormones and thyroid-stimulating hormone (TSH) will be assessed.
Once you have established your horse’s iodine status, consult with an equine nutritionist to appropriate balance the diet.
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