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When a Hypothyroid Dog Really Isn't

Pet Column for the week of September 14, 2009

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Office of Public Engagement
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Ashley Mitek
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Source - Thomas K. Graves, DVM, MS, PhD, DACVIM
Rover, an 8-year-old Labrador retriever, shows up at his veterinarian's office for an annual check-up. After one too many table scraps he is slightly overweight and perhaps, in his older age, not as rambunctious as a few years ago. Because he is a "geriatric" patient, blood work is recommended.

When one of the values on the chemistry panel (a specific type of blood test that looks at organ function) comes back low, the doctor explains that Rover has hypothyroidism. This means that the dog's body is not producing enough of a certain hormone that controls many functions, most notably metabolism, potentially explaining the weight gain and lethargy.

While Rover may in fact be hypothyroid, the disease cannot be accurately diagnosed by a low TT4 (thyroxine), the type of measurement commonly found on a routine chemistry panel for geriatric patients.

Dr. Thomas Graves is a veterinary internal medicine specialist and an endocrinology expert at the University of Illinois Veterinary Teaching Hospital in Urbana. He says, "obesity is very common in dogs and people tend to think about hypothyroidism as a potential cause." But not every dog that is overweight is hypothyroid. In fact, there are very few dogs that are truly plagued by the disease.

"There are many reasons why a dog's TT4 on a chemistry panel may be low," notes Dr. Graves. For one, almost any type of illness can drop the levels below normal and various commonly used drugs can also have the same effect.

Diagnosing a hypothyroid dog is not straightforward. A clinician must take into account all of the patient's clinical signs and blood work values, including cholesterol levels and specific types of testing that measure Free T4 and Thyroid Stimulating Hormone (TSH) levels. In addition, the patient must be showing clinical signs of the disease such as: inactivity, obesity, decreased appetite, skin abnormalities, mental dullness, and intolerance to exercise and cold.

The reason it is a problem that dogs are erroneously diagnosed with hypothyroidism is because the standard treatment, thyroid hormone supplementation, might have negative effects. "We know that anytime you give a hormone it affects several other body systems," explains Dr. Graves. But since no one has studied what exactly happens when you over-supplement with thyroid medication, researchers don't know what harm it is causing.

"To me, the greatest problem with giving a dog thyroid replacement therapy without a proper diagnosis is the fact that you may allow an underlying disease to progress undiagnosed," notes Dr. Graves. Since administering thyroid hormone to dogs often makes them more "peppy," and overall, look healthier, it is possible that you will mask another ongoing disease.

It is interesting to note that in contrast to dogs, hypothyroidism in humans is quite common and easy to diagnose. The American Association of Clinical Endocrinologists estimates that approximately 25 million people have the disease, though half of them will never be diagnosed. Until veterinary experts can develop a better way of testing dogs, hypothyroidism will continue to be a frustrating disease to diagnose, requiring special blood tests interpreted by experienced clinicians.

For more information about canine hypothyroidism, contact your local veterinarian.