Trace Mineral Basics: Iodine

When we think of iodine, most of us think of the dark yellowish-brown liquid in the first-aid kit we might use to treat our horses’ wounds. Indeed, povidone-iodine is a common skin antiseptic. But iodine (I) itself is actually an important trace mineral in the horse’s diet.

Iodine is concentrated in the thyroid gland, located next to the trachea and behind the larynx (voice box). It is essential for the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate the body’s metabolism.

Requirements & Sources

According to the National Research Council’s Nutrient Requirements of Horses (NRC), 2007 edition, an average mature 1,100-pound (500-kilogram) horse requires a minimum of 3.5-4.5 milligrams of dietary I per day, depending on exercise regimen and breeding status.

Little information is available regarding iodine concentration of common forages, because it’s not usually measured in sample analyses. Iodized salt is a simple way to supplement I. Molasses, alfalfa meal, whey, algae, and kelp also contain I.

Deficiency and Toxicity

Goiter, an enlargement of the thyroid gland, is a classic symptom of both I deficiency and toxicity. General signs of deficiency include lethargy, dull coat/hair loss, lack of appetite, thickened skin, and cold intolerance. Deficient foals can be stillborn or born weak, have hypothermia, difficulty standing, and a poor suckle response. If not corrected, I deficiency can lead to developmental orthopedic disease and problems in multiple organ systems.

Iodine toxicity is a possible problem with over-supplementation. Interestingly, goiter can also appear as a result of excess I intake. Greater disease susceptibility, and the potential for chronic respiratory disease, low-grade fever, constant eye tearing and nasal discharge that doesn’t respond to treatment are signs of excessive I intake.

Take-Home Message

While only required in very small amounts, I is crucial for healthy equine metabolism, proper growth and development, and overall health. Because it isn’t typically measured in hay or forage analyses, it’s difficult to estimate I concentration in these feedstuffs without specifically testing for it. Most commercial grain concentrates are fortified with I. For horses consuming forage-only diets, a trace-mineralized salt block with iodine is an easy way to encourage sufficient intake.