Armed Forces Institute of Pathology
Goiter Belt USA
Goiter Belt USA
[OU1] This interaction among the Hypothalamus and Pituitary in the brain with the Thyroid gland balances the horse’s metabolism and growth.
Goiter Belt in the United States
The north-central section of the United States was named the Goiter Belt in the early 1900s. From the Rockies to the Great Lakes and east to western New York, it was discovered that there was a higher incidence of enlarged thyroid glands in armed forces recruits during World War I. The lack of iodine in the soil and water caused the goiters to develop. As iodized salt became readily available, goiters became less common.
Horses are susceptible to goiters caused by iodine deficiency. Most horses with goiters are euthyroid—have normal thyroid function. If you live in the Goiter Belt, should you be concerned for your horse?
Is it Goiter, Hypothyroidism, or Equine Metabolic Syndrome/Insulin Resistance?
Goiters are usually caused by a lack of trace mineral iodine. Excess iodine can also cause goiters. A goiter appears as swelling in the throatlatch area. The lumps are the swollen lobes of the thyroid gland.
“Easy keepers” have often been branded as hypothyroid with low thyroid function, but comprehensive testing for T3, T4, and TSH should be done before medicating for low thyroid. If you treat a horse for low thyroid based on symptoms alone, the drug treatment can actually cause hypothyroidism by suppressing normal thyroid function.
Equine Metabolic Syndrome, EMS, is sometimes confused with hypothyroidism. Horses with EMS seem to gain weight while eating nothing but low-quality hay. They are prone to laminitis at the end of summer going into fall. This happens as they are “bulking up” for the lack of food that could be absent in winter. These horses are also prone to cold induced laminitis when winter temperatures drop.
That strong relationship within the Hypothalamic Pituitary Thyroid Axis affects all three organs. As Dr. Eleanor Kellon, VMD, writes, “an under-functioning thyroid does not cause insulin resistance or Cushing’s Disease, but may be caused by them.”[2][OU2]
Prevention/Treatment of Goiters
If your horse starts to develop a goiter, it’s important to start treatment promptly. Since goiter can be caused by too much or too little iodine, you’ll have to evaluate the iodine in your horse’s diet. Blood plasma levels of thyroid hormones can fluctuate, so it’s difficult to determine iodine excess or deficiency through a blood test.[3][OU3]
Goitrogenic feeds can hinder iodine absorption into the thyroid gland. Avoid feeding raw soybeans, kale, white clover, turnips, rutabagas, and broccoli. The heating of soybeans to make soybean meal eliminates the goitrogenic properties of soybeans. Weeds can also be goitrogenic. Nitrate accumulators such as mustard weed, lambs quarter, curlydock, sorghum-Sudangrass, and redroot pigweed are goitrogenic as well as potentially poisonous. If your horse is eating enough iodine but develops a goiter, check your pasture or forage for these plants.
If you have been feeding kelp, iodine overdose is probably the culprit. Kelp has various amounts of iodine up to 66 mg in 1 ounce (28 grams). A moderately working 1,100-pound horse requires 4 mg iodine per day. Pregnant mares have experienced iodine toxicity at 48-55 mg per day. [4][OU4] The Guide to Equine Supplements and Nutraceuticals recommends keeping iodine daily dose at under 35 mg per day for pregnant mares. [5][OU5]
Both excess and deficient iodine can adversely affect unborn foals. Congenital Hypothyroidism and Dysmaturity syndrome in foals occurs sporadically in Western Canada, the Pacific Northwest and other areas of the United States. These foals are born after prolonged pregnancies with goiter and symptoms of prematurity. Clusters of foals have been affected on a farm in one year without repetition in subsequent years. Causes have not been determined, but it seems that diets low in iodine, high in nitrates with possible ingestion goitrogenic plants are the origin. [6][OU6]
Since iodine is a necessary micronutrient, most commercial feeds contain iodine. Salt blocks may contain iodine. Some well water has iodine. If you suspect your horse may have iodine overdose be aware of any topical products -- shampoos, hoof dressings, wound care -- that contain iodine as it can be absorbed through the skin.
Other Symptoms of Iodine Toxicity or Deficiency
This micro-nutrient imbalance can also present with other symptoms for your horse.
- Hairless foals
- Dry flaky skin
- Thickened skin on the lower legs
- Dry dull hair
- Hair loss
- Low body temperature and cold intolerance
- Lethargic, dull, timid behavior
Use of Alternative and Natural Treatments as Iodine Levels Normalize
The most natural source of iodine is kelp. It’s important that the kelp supplement you use has been tested for the iodine content, so you don’t overdose. If you are feeding commercial feeds or supplements, be sure to add the iodine content of those products at the rate you are feeding into the total.
Some horses can be fed kelp free choice as they seem to eat only what they need. If your horse is Insulin Resistant and eats everything available, you may want to offer her only a daily ration by choice instead of top dressing or mixing the kelp with her concentrate.
The suggested daily dose for a 1,000 pound horse at maintenance is 1 to 6 mg iodine per day. One Tablespoon Iodized table salt contains 135 micrograms (0.135 mg) iodine. In order to get 1 mg iodine, your horse would have to consume 7.4 Tablespoons, which would surely be more sodium than he needs or would probably tolerate!
Forage and water sometimes contain iodine. If you’re using municipal water, see if they have statistics for iodine levels. Well water can contain iodine, so be sure to get a test that includes iodine levels. Forage usually contains little or no iodine, but if your horse is affected by goiter, you may want to have your hay and/or pasture tested.
Applied Kinesiology Muscle Testing can be used to determine whether a supplement or feed source will help or hurt your horse’s condition. This process is described in Complete Holistic Care and Healing for Horses. [7][OU7]
Balance is Key to a Healthy Horse
Macro and micronutrients are vital to your horse’s overall health. The balance of these nutrients plays a large part in your horse’s soundness and stamina as well as athleticism. Before adding a supplement to your horse’s diet, make sure it is necessary. Your horse and bank balance will thank you.
References
[1] Eleanor Kellon, VMD Equine Cushing’s and Insulin Resistance 2008 Chapter 1 pg 3
[2] Eleanor Kellon, VMD Equine Cushing’s and Insulin Resistance 2008 Chapter 1 pg 14
[3] Karen Briggs, Equine Nutrition 2007 pg 89
[4] Baker, H.J. and J.R. Lindsey. 1968. Equine goiter due to excess dietary iodine. J. Am. Vet. Med. Assoc. 153:1618.
[5] Eleanor M. Kellon, VMD Horse Journal™ Guide to Equine Supplements and Nutraceuticals pg 126
[6] http://www.merckvetmanual.com/endocrine-system/the-thyroid-gland/non-neoplastic-enlargement-of-the-thyroid-gland#v4734086 By Janice E. Kritchevsky, VMD, MS, DACVIM, Professor, Large Animal Medicine, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University
[7] Mary L. Brennan, DVM Complete Holistic Care and Healing for Horses 2001 pg 148-150
Goiter Belt Image from Armed Forces Institute of Pathology: The Goiter Belt in the United States has low levels of Iodine in the soil and water
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