Dr. Getty’s June Tip: Iodine Keeps Thyroid Working Properly
A full-sized horse does best on a diet that offers 1 to 6 mg of iodine each day to keep his thyroid gland working properly. Because the iodine content of grass is too low to measure, it is best to rely on supplementation -- from salt or other sources -- to meet your horse's need. Many supplements and fortified feeds already add it. It's always best to know what your horse is consuming since too much iodine can damage the thyroid gland.
Since all full-sized horses require at least one ounce (2 tablespoons) of salt per day for maintenance (and up to 3 ounces/day when perspiring heavily), iodized salt is a good way to add iodine and provide the needed salt as well. Granulated salt that you buy in the grocery store comes in both non-iodized and iodized versions; one teaspoon of iodized table salt contains 0.4 mg of iodine (3 tsp = 1 Tablespoon = 15 ml).
White and brown salt blocks generally do not contain iodine, whereas blue and red ones do. Sea salt, kelp, and other natural salt sources can vary tremendously in their iodine content. Only use reputable sources that guarantee their iodine analysis in writing.