The trace mineral iodine is regarded by some as one of the most critical dietary trace minerals. This is due to the important role that it plays in the thyroid metabolism and in the synthesis of the thyroid hormones triiodothyronine and thyroxine. These hormones fulfill multiple functions ranging from cell regulation to tissue differentiation and growth. When the body’s iodine status declines to the point that the levels of these hormones become insufficient, the thyroid gland becomes enlarged. This phenomenon is commonly referred to as goiter.

Some of the earliest references to goiter comes from the ancient civilizations of China, India, Greece, and Rome. In particular, a book of Chinese origin dating back to 2000 BC, referenced the use of seaweed against goiter. Iodine was discovered in 1811 and, following the detection of iodine in seaweed, a Swiss physician made the connection and started treating patients suffering from goiter with iodine solutions around 1820.

Globally, almost all countries have been affected by iodine deficiency disorders at some point in time. In part this relates to the fact that soil iodine concentrations tend to be low in general; this is often reflected in plant iodine concentrations. Therefore, iodine deficiencies tend to be more prevalent than toxicity in unsupplemented grazing animals.

Adding iodized salt is an efficient way to increase iodine in both human and animal diets. Having a free-choice iodized salt block available to your horse provides him or her an avenue for I intake. If you live in an area known to be iodine-deficient (also referred to as goitrogenic areas), local feed manufacturers might include iodized salt in equine concentrates or you can add a set amount of iodized salt to your horse’s grain or concentrates. Additionally, some owners add a seaweed- or kelp-based supplement to their horses’ diets; as always, it’s important to following the manufacturer’s feeding guidelines for such products.

Fortunately, a horse does not require iodine in large quantities. A mature horse weighing 500 kilograms (1,100 pounds), at rest or performing light exercise, only requires 3.5 milligrams of iodine per day (National Research Council’s Nutrient Requirements of Horses, 2007). This amount increases for mares in late gestation, lactating broodmares, and horses in heavy work.

Should you have any concerns or questions regarding your horses’ iodine intake, a horse nutritionist will be able to assist you in evaluating your horses’ diet.

Mieke Holder, PhD, is an assistant research professor within the University of Kentucky’s Department of Animal and Food Sciences.