Unlike many of the other minerals, iodine (I) has only one main function in your horse's body...
...it is an important part of thyroxine (T₄) and triiodothyronine (T₃).
Both (T₃) and (T₄) are thyroid hormones.
And they aren't just any thyroid hormones...they are the two major hormones that regulate basal metabolism.
In tissues throughout your horse's body, as well as in the thyroid gland itself, T₄ is turned into T₃. If either of these hormones are found in excess, the amount of TSH (Thyroid Stimulating Hormone) is decreased. Not only is TSH decreased, but the body's basic metabolic rate is increased.
Most common feedstuffs fed to horses contain a concentration between 0-2 mg/kg dry matter.
The concentration can fluctuate quite a bit even within the same feed because feed concentration depends on the concentration in the soil in which the feed was grown.
Kelp (pictured above) and other seaweeds can have very high concentrations, sometimes as high as 1850 mg/kg dry matter!
Similarly, ethylenediaminedihydroiodide (now THAT's a mouthful!), more commonly known as EDDI, has high levels of of this mineral and can cause excess to be taken in by the horse. EDDI is commonly used as an anti-fungus supplement in horses.
For horses, iodine is usually supplemented in the diet by providing iodized salts, or salt with added trace-minerals. This is an acceptable way for horses to obtain salt, but be sure to monitor their intake.
Horses have been known to become iodine-deficient when the only supplemental source in the diet is through iodized salt.
In recent years, excesses have been more common than deficiencies.
Toxicities or excesses are only likely to occur when your horse is being over-supplemented with the mineral, or being fed feedstuffs that contain an unusually large amount.
Likewise, when horses are fed large amounts of kelp or seaweed, a toxicity can occur.
Another way that this mineral is unique is that both an excess and deficiency cause the same symptoms!
When there is a deficiency, not enough of the thyroid hormones can be produced. This makes sense, because if you don't have the ingredients, you can't properly make something.
Since there aren't enough hormones being produced, TSH production increases to attempt to increase the production of the hormones.
As a result of the TSH increase, the thyroid works harder. Due to the extra work, the thyroid increases in size, and your horse ends up with a condition known as goiter, or enlarged thyroid.
On the other hand, if there is an excess, the extra mineral present inhibits the production of the two hormones...
...see where this is going?
You got it!
Not enough hormones being produced means that TSH production increases. Increased TSH production means a harder working thyroid, which leads to a larger thyroid...
...and yet again, your horse ends up with an enlarged thyroid, or goiter.
To my knowledge, this is the only mineral in the horse diet that an excess or deficiency cause the exact same symptom. The good news is, you only have one symptom to worry about!
But, on the other hand, if your horse does end up with goiter, you'll have to determine whether it is a deficiency or excess causing the problem.
A measure of the iodine found in the urine can estimate how much a horse is consuming.
Goiter is not the only symptom of excess or deficiency.
In broodmares, the vitality of foals is decreased in the presence of either a deficiency or excess. Deficiency can also result in stillborn foals, as well as abnormal estrous cycles in mares.
Excess can also cause an increase in susceptibility to infections in horses of any age.
Iodine is a very unique mineral in the equine diet. It is the only mineral that has only one function in the body, as well as the only mineral that both a deficiency and excess cause the same symptoms!
Knowledge of this unique mineral will help you more easily understand what it does for your horse, and how he obtains it in his daily diet, as well as deficiency and toxicity symptoms.