Iodine is used in the body for the production of thyroid hormones, T3 and T4. These hormones play a major role in regulating the metabolism of the horse. Ingested iodine is converted in the digestive tract to iodide and absorbed into the body. The thyroid gland, lactating mammary gland and placenta are able to concentrate iodide from the extracellular fluid for use by the thyroid gland itself, for the nursing foal and the developing fetus. This concentrating ability is an advantage when iodine dietary levels are low but creates potential for toxicity when levels are high.
According to nutritionist Lon D. Lewis, soils in areas of recent glaciation, in areas distant from the sea, and in areas of low annual rainfall are most likely to be iodine deficient. The good news is that iodine is required in fairly low levels and is easily supplemented. The bad news is that the symptoms of deficiency and toxicity are exactly the same so one has to know if too much or too little iodine is the problem.
Symptoms of iodine deficiency or toxicity include:
- hypothyroidism, goiter or hypertrophy of the thyroid gland
- a dry, dull hair coat
- hair loss
- flaky skin
- thickened skin of the lower legs
- birth of hairless foals
- lethargy, dullness, and timid behavior
- old intolerance and low body temperature
Hypothyroidism is the most common problem with both deficiency and toxicity of iodine. Most other symptoms are related to low thyroid function . Inadequate levels of iodide limit the production of thyroid hormones and high levels have a direct suppressing effect on the thyroid hormone release by the gland. When these hormone levels drop in the blood the brain sends out more thyroid stimulating hormone but with too little or too much iodide the gland grows in size but is not able to produce or release adequate hormones. This is what, over time, causes the goiter or hypertrophy of the gland. A horse can have hypothyroid symptoms without having an enlarged thyroid gland.
Although true hypothyroidism is caused by iodine deficiency or excess, this is not the only cause of hypothyroidism in the horse. Hypothyroid symptoms in the horse can be secondary to insulin resistance . Insulin resistance, or other conditions that trigger the production of excessive levels of free radicals, increase the body’s need for the same enzymes required to make thyroid hormones active. With proper dietary treatment for insulin resistance the secondary hypothyroid symptoms will be addressed as well.
NOT ENOUGH OR TOO MUCH?
Determining whether there is a deficiency or excess of iodine requires a bit of detective work. First you want to determine if you are in one of the areas that is low in iodine. Remember that prolonged drought can lower iodine in areas that normally have plenty. Certain plants can also interfere with the uptake of iodine into the thyroid gland. These plants are classified as goitrogenic and include kale, white clover, turnips, rutabaga, and broccoli. Author Pat Coleby from Australia also includes alfalfa, beans, peas, and soy products as goitrogenic foods. Coleby made the observation with her goats that feeding too much goitrogenic foods resulted in increased ratios of male to female kids. The female fetus has greater requirement for iodide than the male so females die in utero while the males go on to survive. I also noticed this phenomenon in my own goatherd during severe drought when I had 7 boy kids and only one girl. Other breeders in the area reported similar occurrences.
Toxic levels of iodine do not exist in natural soil but some plants such as capeweed, which are high in nitrates, can interfere with iodine absorption. These plants will not usually be eaten by animals if other forage is available.
Improper supplementation can actually create iodine toxicity. For a 1000 pound horse, feeding more than 4 to 8% iodized or trace mineral salt to the total diet would require 8 to 16 ounces of salt; this would be unlikely but possible. Kelp or seaweed supplements, on the other hand, require much lower amounts to provide excess iodine. As little as 0.7 ounces a day could be harmful, so seaweed supplements should always be offered free choice rather than top dressed on the feed for any extended period.
Avoid feeding excessive amounts or multiple iodine or iodide salts of potassium or sodium or commercial supplements that contain them. Avoid feeding more than 40-50mg/horse/day of organic iodine such as EDDI (ethylene diaminedihydroiodide). This product is sometimes added to the diet or given intravenously for short periods to treat stubborn respiratory conditions.
One possible overlooked source of iodine toxicity is the topical use of iodine containing hoof treatments and shampoos. These products can be absorbed through the skin especially if mixed with DMSO. Occasional use is not a problem, but repeated application of these products for weeks at a time can result in flaky skin and other iodine toxicity symptoms.
If you see symptoms you feel could be related to iodine levels, determine first if any of the sources of toxic levels are present. If you don’t see where toxic levels are likely then offer free choice quality seaweed (kelp), iodized salt or trace minerals. These products should address any deficiency within a few weeks unless your horse is severely malnourished or suffering from a chronic disease or injury.
If you horse is not deficient he is not likely to eat the supplements. If you don’t see improvement then make sure and check your horse’s diet for foods that are goitrogenic or that interfere with iodine absorption.
Madalyn Ward, DVM, owns Bear Creek Veterinary Clinic in Austin, Texas. She is certified in Veterinary Homeopathy and Equine Osteopathy. Memberships include American Veterinary Medical Association, American Association of Equine Practitioners, American Holistic Veterinary Medical Association, Texas Veterinary Medical Association and the Academy of Veterinary Homeopathy. She has authored several books and publishes the monthly newsletter, “Holistic Horsekeeping.” www.holistichorsekeeping.com , www.yourhorsebook.com