Iodine in Horse Nutrition: Why Deficiency and Toxicity Both Matter

Iodine is one of those trace minerals that doesn't get a lot of attention in the horse world — until something goes wrong. Unlike iron or calcium, which horse owners tend to think about more often, iodine quietly plays a critical role in your horse's metabolism. The tricky part? Both too little and too much iodine can cause serious health problems.

This article covers everything you need to know about iodine in your horse's diet: what it does, how much your horse needs, what happens when levels are off, and how to make sure you're getting it right.

What Does Iodine Do in the Horse's Body?

Iodine's primary job is to support the thyroid gland, a small but mighty gland located in the throat area. The thyroid produces two key hormones — thyroxine (T4) and triiodothyronine (T3) — and both require iodine to be synthesised.

These thyroid hormones regulate:

• Metabolic rate — how efficiently your horse converts feed into energy
• Growth and development — particularly important in foals and young horses
• Reproductive function — crucial for breeding mares and stallions
• Thermoregulation — the ability to maintain body temperature
• Skin and coat health — thyroid hormones influence hair growth cycles

Without adequate iodine, the thyroid can't produce enough of these hormones, and your horse's entire system starts to slow down. But flood the body with too much iodine, and you paradoxically get a very similar set of problems.

How Much Iodine Does a Horse Need?

The National Research Council (NRC) recommends the following daily iodine intakes for horses:

• Adult horses at maintenance: approximately 3.5 mg per day (for a 500 kg horse)
• Pregnant and lactating mares: approximately 4–4.5 mg per day
• Growing foals and young horses: approximately 2.5–3.5 mg per day, depending on age and weight

The NRC sets the upper safe limit at around 5 mg per day for a 500 kg horse, though some sources suggest that chronic intake above 40–50 mg per day is where clinical toxicity becomes a real concern.

That might seem like a wide margin of safety, but it's surprisingly easy to overshoot when multiple supplements, feeds, and mineral blocks all contain added iodine — especially in the form of kelp.

Where Does Iodine Come From in the Diet?

Forage and Pasture

The iodine content of hay and pasture depends heavily on the soil where it was grown. In many parts of the world — including large areas of the UK, Europe, Australia, and parts of North America — soils are naturally low in iodine. This means the forage your horse eats may contribute very little to their daily requirement.

Coastal regions tend to have slightly higher soil iodine levels, but even then, you can't rely on forage alone to meet your horse's needs.

Commercial Feeds and Balancers

Most commercial horse feeds and ration balancers include supplemental iodine, typically in the form of calcium iodate or potassium iodide. If you're feeding the recommended amount of a well-formulated feed, it's likely contributing a meaningful portion of your horse's iodine requirement.

Salt and Mineral Blocks

Iodised salt is a common source of supplemental iodine. Standard iodised table salt contains approximately 0.006% iodine, so a horse consuming 30–50 g of iodised salt daily picks up a small but useful amount.

Mineral blocks and loose mineral mixes also frequently contain added iodine.

Kelp and Seaweed Supplements

This is where things get interesting — and potentially dangerous. Kelp (seaweed) is an extremely rich natural source of iodine. Depending on the species and where it was harvested, dried kelp can contain anywhere from 200 to over 2,000 mg of iodine per kilogram.

A 30 g daily serving of a high-iodine kelp could deliver 6–60 mg of iodine — potentially well above safe levels. This makes kelp one of the most common causes of iodine toxicity in horses.

Iodine Deficiency in Horses

Causes

• Low-iodine soils producing forage with minimal iodine content
• Unsupplemented diets — horses on forage-only diets with no mineral supplementation
• Goitrogenic compounds — certain feeds, including some brassicas (kale, turnips, rapeseed) and clover, contain substances that interfere with iodine uptake by the thyroid, increasing the horse's effective requirement

Signs of Iodine Deficiency

Iodine deficiency leads to hypothyroidism — an underactive thyroid. In horses, this can manifest as:

• Goitre — an enlarged thyroid gland, visible as swelling in the throat area. This is the classic sign.
• Poor coat quality — a dull, rough, or slow-growing coat
• Weight gain or difficulty losing weight despite appropriate feeding
• Lethargy and reduced performance
• Cold intolerance
• Reproductive problems in mares, including irregular cycles, difficulty conceiving, and prolonged gestation

Iodine Deficiency in Foals

This is where iodine deficiency becomes particularly devastating. Mares with inadequate iodine during pregnancy may give birth to foals with:

• Congenital goitre — sometimes severe enough to cause breathing difficulties
• Weakness and inability to stand or nurse
• Limb deformities or contracted tendons
• Stillbirth or early neonatal death

These outcomes are tragic and largely preventable with proper mineral nutrition during pregnancy.

Iodine Toxicity in Horses

Here's the paradox that catches many well-meaning horse owners off guard: iodine toxicity produces many of the same symptoms as deficiency, including goitre. Excess iodine disrupts normal thyroid function just as effectively as a shortage does.

Causes

• Over-supplementation with kelp or seaweed products — by far the most common cause
• Stacking multiple supplements that all contain iodine without realising the cumulative dose
• Feeding iodised salt alongside an iodine-rich supplement and a fortified feed

Signs of Iodine Toxicity

• Goitre — yes, the same enlarged thyroid seen in deficiency
• Excessive tearing and nasal discharge
• Dry, flaky skin and a rough coat
• Coughing or respiratory issues
• Decreased appetite
• Reproductive problems — including foal abnormalities similar to those caused by deficiency

In severe or chronic cases, iodine toxicity can lead to thyroid suppression, where the gland essentially shuts down in response to the iodine overload. This creates a state of hypothyroidism even though the horse is consuming excess iodine — a confusing clinical picture.

Toxicity in Foals

Pregnant mares consuming excessive iodine — often through well-intentioned kelp supplementation — can produce foals with severe thyroid dysfunction. Neonatal goitre from maternal iodine excess has been documented multiple times in veterinary literature and is a genuine risk.

The Kelp Conundrum

Kelp is widely marketed as a natural, nutrient-rich supplement for horses. And it does contain a broad range of trace minerals. The problem is the iodine content, which can be extraordinarily high and highly variable between batches.

If you choose to feed kelp:

• Look for products that declare the iodine content per serving — and do the maths
• Feed very small amounts and account for all other iodine sources in the diet
• Avoid kelp entirely for pregnant mares unless you have precise iodine figures and have calculated total dietary iodine carefully
• Don't assume "natural" means "safe at any dose" — iodine toxicity from kelp is well documented

For most horse owners, a properly formulated mineral supplement or ration balancer is a much safer and more predictable way to meet iodine requirements.

How to Get Iodine Right in Your Horse's Diet

Getting iodine nutrition right requires looking at the whole diet rather than guessing. Here's a practical approach:

Step 1: Know What You're Feeding

List every source of iodine in your horse's daily diet. This includes:

• Hard feed or balancer (check the label for iodine content per serving)
• Mineral supplements
• Salt (is it iodised?)
• Kelp or seaweed products
• Any other supplements that may contain iodine

Step 2: Add Up the Total

For a 500 kg adult horse, aim for a total daily intake of approximately 3–5 mg of iodine. If you're consistently exceeding this, consider removing or reducing one source.

Step 3: Analyse Your Horse's Full Diet

The most reliable way to ensure your horse is getting the right amount of iodine — alongside every other mineral and nutrient — is to properly analyse your horse's complete diet. Tools like the one available at myequibalance.com/analyse allow you to input all your feeds and supplements and see exactly where your horse's nutrition stands. This takes the guesswork out of trace mineral balancing and helps you avoid both deficiency and excess.

Step 4: Be Extra Careful with Pregnant Mares

If you have a broodmare, iodine balance is non-negotiable. Neither deficiency nor toxicity is acceptable. Work with a nutritionist or use a detailed diet analysis to ensure her iodine intake is within the recommended range throughout pregnancy and lactation.

Step 5: Don't Double Up Without Knowing

One of the most common mistakes is feeding a ration balancer that already contains adequate iodine and then adding a kelp-based supplement or an additional mineral mix on top. Always check for overlap.

When to Suspect an Iodine Problem

Consult your vet if you notice:

• Swelling in the throat or jaw area (possible goitre)
• Unexplained poor coat quality, lethargy, or weight changes
• Reproductive difficulties in a breeding mare
• A newborn foal with breathing difficulties, weakness, or visible throat swelling

Your vet can run blood tests to assess thyroid hormone levels (T4 and T3) and help determine whether iodine imbalance may be a factor. It's worth noting that thyroid testing in horses can be nuanced — other factors, including stress, illness, and diet, can influence results — so interpretation should always be done in context.

Key Takeaways

• Iodine is essential for thyroid function, metabolism, growth, and reproduction in horses.
• Deficiency and toxicity produce similar symptoms, including goitre and reproductive failure — making it crucial to get the dose right rather than simply adding more.
• Kelp and seaweed supplements are the most common cause of iodine toxicity in horses due to their extremely high and variable iodine content.
• The margin between adequate and excessive intake is relatively narrow, especially when multiple supplements are in use.
• Pregnant mares are particularly vulnerable — both deficiency and excess can cause devastating outcomes in foals.
• Analyse your total diet rather than guessing, and always account for every source of iodine your horse receives.

Iodine is a perfect example of why "more is not better" in equine nutrition. With a little attention to detail and a willingness to do the maths, you can ensure your horse gets exactly what it needs — no more, no less.