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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.
Foals are most at risk of iodine deficiency or excess. Foals born to mares that consume too much or too little iodine can experience growth abnormalities and general weakness. [1][2]
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). [10] 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.
What's your top priority with your horse's health?
What's your top priority with your horse's health?
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. [11]
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. [4]
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 dihydride (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 ounces (30 – 60 grams) of salt per day. Using iodized salt will provide 1.34 – 2.68 mg of iodine per day.
Calcium iodate (inorganic form) and EDDI (organic form) are commonly added to commercial feeds. These have almost equivalent bioavailability, based on studies in other animals. [6][7]
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. [8][9]
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: [3]
- Goitre
- Rough coat
- Dry skin
- Lethargy
- Exercise intolerance
Primary hypothyroidism is extremely rare in horses. However, sick horses often have low thyroid hormone levels, which is referred to as euthyroid sick syndrome. Euthyroid sick syndrome is expected to resolve when the underlying disorder is treated.
Iodine Toxicity
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 / 1,100 lb horse is 50 mg per day. However, as little as 25 mg/day in pregnant mares can cause extremely large goitres in newborn foals.
Signs of iodine toxicity include: [3]
- Goitre
- Weight loss
- Tachycardia (rapid heart rate)
- Tachypnea (rapid breathing)
- Ravenous appetite
- Alopecia (hair loss)
- Hyperreactivity
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.
Urine iodine levels can be used to assess whether your horse is consuming too much or too little iodine. [3][5]
Once you have established your horse’s iodine status, consult with an equine nutritionist to appropriately balance the diet.
Frequently Asked Questions
Here are some frequently asked questions about iodine in the horse’s diet:
Iodine is an essential mineral that horses need to produce thyroid hormones, which regulate metabolism, growth, and normal function throughout the body. More than 75% of the body's iodine is stored in the thyroid gland, where it is used to make hormones that influence heart function, digestion, muscle development, skeletal growth, and energy use in nearly every tissue.
An average 500 kg (1,100 lb) horse requires approximately 3.5 to 4 mg of iodine per day to meet maintenance needs. Requirements are based on dry matter intake, with horses needing about 0.35 to 0.4 mg of iodine per kilogram of dry matter consumed. Growing horses, broodmares, and horses with different feeding programs may have different dietary requirements.
Hay alone is unlikely to provide enough iodine to meet a horse's daily requirement. Average hay contains relatively low levels of this mineral, and a horse consuming 10 kg (22 lb) of hay may only obtain about 1.3 mg of iodine per day. Many horses require additional iodine from fortified feeds, supplements, or iodized salt.
Common dietary sources of iodine include iodized salt, calcium iodate, EDDI, kelp meal, fortified feeds, and vitamin and mineral supplements. Many commercial products contain added iodine to compensate for naturally low levels found in forage. Horses fed complete feeds or ration balancers at recommended amounts often receive adequate iodine through those products.
Iodized salt can provide a significant portion of a horse's daily iodine requirement while also supplying sodium. Feeding 1 to 2 ounces (30 to 60 grams) of iodized salt per day provides approximately 1.34 to 2.68 mg of iodine. Total iodine intake still depends on the rest of the diet, including forage and any fortified feeds or supplements.
Iodine deficiency can lead to reduced thyroid hormone production and signs of thyroid dysfunction. Common signs include an enlarged thyroid gland called goitre, dry skin, a rough or dull coat, lethargy, reduced stamina, poor appetite, and exercise intolerance. Severe iodine deficiency is uncommon, but subtle signs may develop when dietary intake remains low for extended periods.
Excess iodine can disrupt normal thyroid function and cause many of the same problems associated with deficiency. Signs of iodine toxicity may include goitre, weight loss, rapid heart rate, rapid breathing, excessive appetite, hair loss, and hyperreactive behavior. Over-supplementation can occur when multiple iodine-containing products are fed together without evaluating total dietary intake.
Goitre is not always caused by iodine deficiency because excessive iodine intake can also enlarge the thyroid gland. Both too little and too much iodine may interfere with thyroid hormone production, making goitre an unreliable way to determine iodine status. Veterinary testing is typically needed to identify the underlying cause of thyroid enlargement.
Foals are particularly vulnerable to both iodine deficiency and iodine excess during development. Mares that consume too little or too much iodine during pregnancy can produce foals with weakness, growth abnormalities, or enlarged thyroid glands. Even moderate excesses can be problematic, which is why broodmare diets require careful mineral balancing.
Kelp meal is a concentrated natural source of iodine and is commonly used in equine diets to increase iodine intake. In addition to supplying this trace mineral, kelp contains compounds that may support hindgut health and act as prebiotics. Because iodine levels can be high, total dietary intake should be considered before adding kelp products.
Thyroid dysfunction may be associated with signs such as lethargy, poor performance, coat changes, weight loss, altered appetite, or thyroid enlargement. These signs are not specific to iodine imbalance and can occur with other health conditions. A veterinarian can evaluate thyroid hormone levels and other diagnostic findings to determine whether thyroid disease is present.
A balanced iodine intake is best assessed by reviewing the entire diet, including forage, salt, supplements, and commercial feeds. Veterinarians can evaluate thyroid hormone levels and, in some cases, urine iodine concentrations to assess iodine status. Working with an equine nutritionist can identify deficiencies or excesses and ensure mineral intake remains within a safe range.
Summary
Providing the right amount of iodine is vital for your horse’s thyroid function, metabolism, and overall health. Keep these key points in mind when managing iodine in your horse’s diet:
- Horses require approximately 0.35–0.4 mg of iodine per kg of dry matter consumed daily
- Forages and grains alone are typically too low in iodine to meet requirements
- Common iodine sources include iodized salt, calcium iodate, EDDI, and kelp meal
- Both iodine deficiency and excess can cause thyroid dysfunction and goitre
- Foals are especially sensitive to iodine imbalances during development
- Consult an equine nutritionist to balance iodine levels and prevent health issues
References
- Drew, B. et al. The effect of excess dietary iodine on pregnant mares and foals. Vet Rec. 1975. View Summary
- Driscoll, J. et al. Goiter in foals caused by excessive iodine. J Am Vet Med Assoc. 1978. View Summary
- Breuhaus, B.A. Disorders of the equine thyroid gland. Vet Clin North Am Equine Pract. 2011. View Summary
- Borucki Castro, S.I. et al. Short communication: Feed iodine concentrations on farms with contrasting levels of iodine in milk. J Dairy Sci. 2011.
- Wehr, Ulrich. Iodine Balance in Relation to Iodine Intake in Ponies. The Journal of Nutrition. 2002. View Summary
- Herzig, I. et al. Utilisation of iodine from different sources in pigs. Archiv für Tierernaehrung. 2000.
- Miller, J.K. and Swanson, E.W. Metabolism of ethylenediamine dihydride and sodium or potassium iodide by dairy cows. J Dairy Sci. 1973.
- Shu, M-H. et al. Anti-inflammatory, gastroprotective and antiulcerogenic effects of red algae Gracilaria changii (Gracilariales, Rhodophyta) extract. BMC Comp Altern Med. 2013.
- Moir, T. et al. The influence of feeding a high calcium, algae supplement on gastric ulceration in adult horses. J Appl Anim Nutr. 2016.
- Michalak, I. and Marycz, K. Algae as a Promising Feed Additive for Horses. Seaweeds as Plant Fertilizer, Agricultural Biostimulants and Animal Fodder. 2019.
- NRC. Nutrient Requirements of Horses. NRC. 2007.
