Hyperthyroidism, a very common condition in cats, is exceedingly rare in dogs. Off the top of my head, I can only remember diagnosing one dog with hyperthyroidism in the course of my career (other than those dogs who were on supplements for hypothyroidism and needed a reduction in dose).
My patient had the classic symptoms of hyperthyroidism: weight loss in the face of an excellent, bordering on ravenous, appetite and increased thirst and urination. Unfortunately, identifying the cause was quite simple. I could easily palpate a large mass on the underside of her neck.
A biopsy confirmed what I suspected; cancer of the thyroid gland.
Until recently, I had thought that cancer of the thyroid gland was essentially the only disease that could cause elevated thyroid hormone levels in dogs, but it turns out that diet can be to blame also. A couple of newly published papers reveal that eating certain types of foods and/or treats puts dogs at risk for dietary hyperthyroidism, which can also be called thyrotoxicosis.
The first study looked at twelve dogs who ate raw meat diets or were fed fresh or dried gullets and had elevated levels of thyroid hormone in their bloodstream.
Half of the dogs had clinical signs such as “weight loss, aggressiveness, tachycardia [an abnormally rapid heartbeat], panting, and restlessness,” while the other half were symptom-free. After changing the diet, the eight dogs that were reevaluated all had normal thyroid hormone levels and any symptoms that were present resolved.
In the next study, researchers identified fourteen dogs who had high thyroid hormone levels while eating commercially available dog foods or treats.
“All 14 dogs were being fed all-meat or meat-based varieties of commercially available dog foods or treats at the time of diagnosis… All samples or descriptions of the suspect foods or treats provided by clients were of a similar” type and included air dried dog foods, jerky treats or strips, and thawed, raw dog food. After four weeks off of these foods or treats, the dogs’ thyroid hormone levels were all back to normal and any symptoms they had were gone.
The suspected cause in all these cases was the inclusion of thyroid tissue in the food or treats being fed to the dogs. A similar problem has been identified in people. Ground beef that inadvertently contained thyroid tissue has led to cases of so-called “hamburger thyrotoxicosis.”
This is a sort of good-news bad-news scenario for owners.
The good news: If your dog develops symptoms and laboratory findings consistent with hyperthyroidism, cancer is no longer the “only” possible diagnosis.
The bad news: We all have to be a little bit more careful about what we choose to feed our dogs.
Dr. Jennifer Coates
Dietary hyperthyroidism in dogs. Köhler B, Stengel C, Neiger R. J Small Anim Pract. 2012 Mar;53(3):182-4.
Exogenous thyrotoxicosis in dogs attributable to consumption of all-meat commercial dog food or treats containing excessive thyroid hormone: 14 cases (2008-2013). Broome MR, Peterson ME, Kemppainen RJ, Parker VJ, Richter KP. J Am Vet Med Assoc. 2015 Jan 1;246(1):105-11.