Hyperthyroidism is extremely rare in dogs. It is typically associated with aggressive thyroid tumors that produce large amounts of thyroid hormone. The only other known cause is the ingestion of thyroid hormone from other sources. In each of the last three years, a research study has documented hyperthyroidism in dogs fed raw diets or treats.
What is Hyperthyroidism?
All animals have thyroid glands. The glands are located next to the trachea (wind pipe) just below the larynx (voice box). These glands secrete thyroid hormone. The amount of thyroid hormone in the blood regulates body metabolism. Decreased levels slow metabolism and increased levels speed up metabolism. Heart rate, body temperature, chemical reactions, food utilization, or storage are all dependent on the level of thyroid hormone in the bloodstream.
Animals with hyperthyroidism secrete excess hormone, causing a constant state of metabolic hyperactivity. They often lose weight, have fast heart rates, and ravenous appetites. Side effects also include increased water consumption, increased urination, and vomiting. Long term, this hyper metabolic state can result in heart and kidney failure.
Cat owners are all too familiar with this condition. Over-active, microscopic benign tumors in the thyroid glands are extremely common in older cats. The condition is so common that a veterinary expert in feline hyperthyroidism once quipped, “It seems that every cat is destined to develop hyperthyroidism at some point in its life.”
What is the Cause of Hyperthyroidism in Dogs Fed Raw Food?
Active thyroid hormone secreting tissue is not restricted to the thyroid gland. Research has shown that small, usually microscopic, amounts of active thyroid tissue can be found along the entire trachea, even into the chest. Dogs fed raw animals necks absorb thyroid hormone from attached or residual thyroid gland or thyroid active tissue in the neck. The amount is sufficient to cause symptoms of hyperthyroidism.
In the 2012 and 2013 studies, the diets of the sick dogs were confirmed to have included raw neck tissue or thyroid gland contamination of raw products from a slaughtering plant. The new 2014 study (unpublished) confirmed beef necks and thyroid tissue in raw dog treats. All of the dogs in the studies had elevated thyroid hormone levels without evidence of thyroid tumors. Dietary change resulted in a return to normal blood thyroid levels and relief from the symptoms, suggesting that the raw thyroid tissue was the underlying cause.
Why Hyperthyroidism in Dogs May Become More Common
The popularity of real food raw diets for dogs is becoming extremely popular. Major ingredients in many of these diets are “meaty bones.” Meaty bones are basically the frame (neck, back, and pelvis) of the chicken or small livestock (rabbits), and necks of large livestock after the majority of the choice muscle has been removed. Chicken necks are a very commonly used meaty bone. The combination of residual meat, ligament, tendon. and bone make them attractive for those choosing to feed a diet that more closely mimics the diet of the wild ancestor of the dog. The high bone content is thought to add adequate calcium, phosphorus, and magnesium, and the necks contribute fat and a small amount of protein to the rest of the diet.
These studies suggest that thyroid tissue contamination of raw animal necks or treats containing neck and thyroid tissue can cause hyperthyroidism in dogs. With larger numbers of dogs being fed raw necks we may see more dogs with this condition.
Fortunately, the condition is reversible once thyroid tissue is removed from the diet. Those choosing to feed a raw diet containing meaty bones may want to avoid using necks as part of the diet. Evaluation of blood thyroid hormone levels in dogs on these diets would also be advisable.
Dr. Ken Tudor