Tips from "Control That Powerful, Fragile Thyroid Gland", Horse Journal Oct 2004. There is not much info in vet textbooks on thyroid function and disease in horses, this article states. Most equine blood tests show T4 (thyroxine) within normal range. Problem is, there can be almost a 400% difference between the upper and lower end of what is considered to be "normal". When analyzing human, canine or feline thyroid, tests are done for T4; T3 (the more active hormone, synthesized from T4); TSH (thyroid stimulating hormone, which provides the most diagnostic test for humans, as it shows how hard the body is prodding the thyroid to produce T4); and free T4 (not bound to protein in the blood, and therefore more available to be read by the cells of the brain). So....if your equine practitioner is not testing all of the above, you are not going to get an accurate "read" on the true thyroid status. It is possible for any species to have a "normal" T4 reading and still have low thyroid activity. It is also important to know what a "normal" reading is for that particular horse. Sometimes symptoms tell the story more than the blood test. Laminitis, insulin problems, Cushing's are all overlapping syndromes that may involve the thyroid, and hypothyroidism can cause tying up via poor utilization of glucose by the muscles. Insulin resistance can be caused by the altered metabolism of cortisol (an adrenal hormone) due to the underactive thyroid. Classic hypothyroidism manifests as overweight, lethargic horses. But weight loss can be a symptom too, especially in Thoroughbreds. Dr. Regan Golob's opinion on this is many horses working hard and as metabolically sensitive as Thoroughbreds have stressed adrenals, which makes the thyroid work even harder to take up the slack. The thyroid may be overactive for a bit (hyperthyroidism) and then crash into hypothyroidism, leaving a depressed, skinny and inappetant TB. Hypothyroid horses can also be reactive, skin sensitive, cinchy, resentful about grooming, and have dry itchy skin and poor hoof quality as well as thinning manes and tails. We also know from an article in 1996 in the New England Journal of Medicine that a congested, toxic and compromised liver makes for inadequate conversion of T4 to T3, 87% of which conversion is done in the liver. So, overuse of chemicals in feeds, drugs, dewormers etc can precipitate hypothyroidism through the mechanism of stressing the liver. A Louisiana State Vet School study showed that bute caused a dramatic drop in T4 and free T4 within four days of the start of bute therapy, and for up to 10 days after stopping the bute. Another test showed a similar effect on T3. Iodine and Selenium are important minerals for thyroid support. Kelp, iodized salt, and other trace mineral supplements provide iodine. This article cites iodine deficiency as the most common cause of hypothyroidism in man and every animal species worldwide. Selenium supplementation needs to be in the range of 1-4 mg per day, depending on activity level of the horse and the area of the country where you live. Hardworking horses may require the upper end of the spectrum. A training level of Dynamite regular provides just under 2 mg of selenium, as our amino acid chelate. TNT likewise, and we also offer Premium E - Selenium for the horses on Dynamite Plus (which has about 1/10 the Se level of regular Dynamite) or who need additional Selenium as determined by blood assay. Additional vitamin E helps the body metabolize Selenium, and that is often the place to start to bring up Se levels. Note that use of MSM at high levels for a considerable amount of time may tie up Selenium. Selenium is needed by the liver enzyme that converts T4 to T3. So, bottom line for me is, do a complete thyroid panel if you are going to blood test. Make sure iodine and selenium are at optimum levels. If you use chemical dewormers, cleanse the liver with Excel or Clay for at least 7 days afterwards. Feed our chemical free feeds and supplements, and hay that is as clean as you can find. Feed at ground level instead of in racks where possible, as reaching up for the hay stresses the thyroid by inverting the cervical curve."