Iodine - Metabolism and Thyroid Function
- Aug 10
- 3 min read
You’ve probably seen kelp in horse supplements, promoted as a cure-all for sluggish horses or metabolic issues. But here’s the catch — when it comes to iodine (which kelp is packed with), more isn’t always better. With some trace minerals, a tiny bit goes a long way.
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).
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 Does Iodine Do?
Iodine helps the thyroid make hormones that regulate:
Metabolism
Body temperature
Growth
Energy levels
Heart health and function
Brain development
Hypothyroidism is rare in horses, however thyroid function is very important to overall health.
Iodine and Insulin Resistance
Thyroid hormones also help regulate blood sugar. So if iodine levels are off and the thyroid slows down, a horse might become less sensitive to insulin — and that means higher risk of:
Weight gain
Laminitis
Trouble managing sugar-rich feed
If a horse doesn’t get enough iodine, the thyroid can’t work properly, which might lead to tiredness, weight gain, or coat issues. But if they get too much iodine — like from overfeeding kelp — it can cause the same problems.
Too little iodine: Poor growth, tiredness, dull coat, sometimes goitre (thyroid swelling)
Too much iodine: Same problems — plus infertility, suppressed thyroid, and even laminitis risk. Symptoms of iodine excess and deficiency look similar, making diagnosis tough without bloodwork. Tolerances in either direction are quite tight.
The National Research Council’s Nutrient Requirements for Horses - which is the gold standard for equine nutrition - states that iodine requirements for horses are approximately 3.5 – 4 mg of iodine per day for a 500 kg horse. Some kelp-based products contain way more iodine than a horse needs — sometimes hundreds of times the daily requirement. It’s important to read the label.
One of the most popular and accessible seaweed meal supplements for horses on the market has a recommended dosage for horses of 30g - 50g per day, which would supply up to 60mg of iodine per day - exceeding minimum requirements many times over, and exceeding the maximum tolerable dose of 50mg/day.
Here at Sound Advice we have a seaweed meal supplement, but we do not recommend its use in conjunction with Sound Advice Trace Mixes, which already contain optimum iodine.
Our seaweed meal supplement is only to be used at the recommended rate of 10g per day. It has a relatively high iodine content compared to most commercial feeds and ration balancers which generally underdose, 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.
We offer seaweed meal in our metabolic bundle for a reason - it’s not for the ordinary horse, but specifically for horses with metabolic dysfunction arising from iodine deficiency.
The amount of iodine found in forages varies by geographic location, proximity to the ocean, plant species and environmental factors. A horse consuming 10 kg of hay would obtain on average 1.3 mg of iodine from the hay. Therefore, hay alone is unlikely to meet your horse’s iodine requirement.
You don’t need to worry about meeting the minimum, or maximum iodine requirements for your horse on Sound Advice Trace Mixes. These mixes also supply optimal amounts of other trace minerals, including copper, zinc and selenium in a 30g scoop.
Supplying trace minerals in the correct proportions is critical because they work in delicate balance with each other, and both deficiency and excess can disrupt vital bodily functions. For example, too much iodine can suppress thyroid function just as easily as too little, while an imbalance in copper and zinc can impair immunity, hoof health, and coat quality.
Horses rely on these microminerals for enzyme activation, hormone synthesis, nerve function, and metabolic regulation, but they only need them in tiny amounts — so small miscalculations can have surprisingly big impacts. That’s why precision matters when designing diets: minerals must be carefully balanced to match the horse’s workload, forage profile, and individual health needs.
Here at Sound Advice, we offer bespoke nutrition for your particular horse. We have three Trace Mix supplements, each designed for different circumstances. Talk to us about which combination of supplements best suits your situation.
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