Hypothyroidism, or Just Plain Obesity?

By: Kim Marie Labak
University of Illinois College of Veterinary Medicine

Hypothyroidism is a disease suffered by both humans and dogs. In dogs, the disease is seen most commonly in middle-aged females. Some dog breeds, such as Cocker spaniels, Golden and Labrador retrievers, Dachshunds, and Doberman pinschers, are predisposed to hypothyroidism, but this condition is extremely uncommon in cats.

In humans and dogs, the hormone T4, produced by the thyroid gland, plays an essential role in regulating growth, metabolism, immune function, and heart function. Deficiency of this hormone, or hypothyroidism, can lead to obesity, decreased energy, and poor skin and hair or coat condition.

According to Dr. Thomas Graves, veterinarian and endocrine specialist at the University of Illinois Veterinary Teaching Hospital in Urbana, most canine cases are caused by Lyphoplasmocytic thyroiditis, an autoimmune disease in which the body produces antibodies that attack the thyroid gland.

According to Dr. Graves, "Canine hypothyroidism is one of the most misdiagnosed dog diseases, and is not as common as we think." In humans, hypothyroidism is quite common and can be diagnosed with 95% accuracy using a simple blood test that has been used since 1965. However, no gold standard test, or group of tests, for canine hypothyroidism currently exists, so hypothyroidism may be difficult to differentiate from other disorders, especially obesity.

Some of the test results associated with hypothyroidism, such as decreased T4 level, increased blood cholesterol, and anemia, can also result from other diseases, making diagnoses complicated. A dog that is simply obese due to overfeeding or lack of exercise will also have a decreased metabolic rate, displaying signs very similar to those of a hypothyroid dog. The only way to get a definitive diagnosis of hypothyroidism is to biopsy the thyroid, but this procedure requires general anesthesia and surgery.

An accurate diagnosis without biopsy requires a skilled, experienced clinician to combine information from a detailed history, a thorough examination, and a strategic combination of tests. Accurate diagnosis is important, since administering T4 treatment to a dog that does not have hypothyroidism may disrupt hormonal balance, and the long-term consequences of this currently remain unknown.

Fortunately, once diagnosed, canine hypothyroidism is simple and inexpensive to treat. Dogs are treated with the same orally administered, synthetic form of T4 as humans. This treatment is very effective, helping a dog regain its energy, and causes no long-term side effects.

To prevent misdiagnosis, Dr. Graves suggests that pet owners prevent obesity, the disease most commonly misdiagnosed as hypothyroidism. "Limiting fattening treats, taking long walks, and playing fetch are good ways to increase and maintain a dog's metabolism and vitality. Any extremely overweight dog needs evaluation by a veterinarian to rule out hypothyroidism or other serious diseases."

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