With the exception of rare and usually benign thyroid tumors in older horses, or thyroid disorders related to dietary iodine, most veterinarians believe changes in thyroid hormone levels in horses are related to other disorders, not to problems with the thyroid gland itself. Results of a study performed by the Faculty of Veterinary Medicine, University of Bari, Italy, may change that.
The researchers examed 662 equine thyroids and found 156 were abnormal (e.g. goiter/enlargement). Of those, 80% of the thyroid glands had microscopic evidence of changes consistent with Hashimoto thyroiditis in people, an autoimmune disease of the thyroid. In addition, circulating autoantibodies to thyroid hormone and thyroid gland enzymes were found in the horses’ blood.
As with all new findings, further studies are needed to confirm this is indeed an autoimmune thyroid disease and to determine how common it might be and any predisposing factors. For now, it raises questions about the wisdom of dismissing enlarged thyroid glands and abnormal thyroid function tests as insignificant.
Autoimmune thyroiditis in other species can cause problems that extend beyond the hypothyroidism it eventually causes. These can include hives, excessive hair growth, insulin resistance and even encephalitis. While thyroid biopsy would be the best way to diagnose the condition, this isn’t without risk because of the rich network of blood vessels and nerves in the area. Examination of the architecture of the gland by ultrasound could also be helpful. Treatment in people is by supplementation with thyroid hormone.
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