by Tom Phillips, D.V.M., MS, PhD.
Canine hypothyroidism is the most commonly diagnosed endocrine (hormonal) disease of dogs, including the Briard. The term hypothyroidism simply means the underproduction of thyroxin, the hormone produced by the thyroid gland. Although this ailment is commonly diagnosed, it is not an easy diagnosis to make, as the thyroid function tests are not definitive and this condition can mimic so many other diseases. It is not the purpose of this article to turn the reader into a thyroid expert, but rather to supply information to assist the breeder and pet owner in gaining a better understanding of this complex disease.
The thyroid gland is located on the trachea (windpipe) of the dog, just below the voice box. The thyroid gland exerts its influence on the dog's body by producing and releasing its hormone (thyroxin) into the blood stream. This hormone and thus, the thyroid gland itself, is very important in controlling growth and development and maintaining normal protein, carbohydrate and lipid metabolism of the dog. Thyroxin is so important that almost every tissue in the dog's body is controlled by the thyroid gland.
The clinical signs of hypothyroidism are not specific and may resemble many other diseases or for that matter even normal states of the dog. Hypothyroidism effects males and females equally and usually occurs between the ages of 2 to 6 years. No one sign is diagnostic of this condition but often a number of clinical signs seen together will prompt a veterinarian to suspect hypothyroidism.
The most common sign of hypothyroidism is increase in body weight, with almost half of the affected dogs demonstrating this sign. Lethargy or mental dullness was seen in 35% of the hypothyroid dogs. The most easily observed abnormalities are seen in the skin, with about one third of the cases demonstrating some form of skin disease (e.g. poor thin coat, large areas of symmetrical loss of hair, thickening of the skin, dandruff, oily skin, roughing and darkening of the skin, development of a "rat tail", increased skin infections, and increased scratching). Twelve percent of the affected dogs showed weakness and a decrease in exercise tolerance, while 4% had grand mal seizures. Reproductive problems such as failure of females to go into season and male infertility are also common. Thus, it is easy to see that hypothyroidism can present in many different forms and can be a difficult disease to diagnose from just the presenting clinical signs.
Diagnosis of hypothyroidism is often not a simple process. Hypothyroidism is often called the great imitator, because it so frequently resembles other disease conditions. Also, the diagnostic tests are not 100% accurate and are often affected by drugs, other hormones, obesity, other skin diseases, systemic diseases, and exposure of the dog to hot or cold conditions. The diagnosis of hypothyroidism is based on clinical history, clinical signs, clinical laboratory results, thyroid function tests, and often the determining factor in making a c6rrect diagnosis is the animal's response to therapy (in other words, does the animal improve when the thyroid medication is given).
The treatment of hypothyroidism, regardless of its cause, is through thyroid hormone supplementation given orally once or twice a day. Usually, if the diagnosis is correct, thyroid supplementation improves the clinical signs associated with the disease within 4-6 weeks. During the first week of therapy, the first improvements are usually seen (an increased mental alertness, greater activity, more muscle strength, and improved appetite. The improvements in the skin and hair take a bit longer - several months to return to normal. All the clinical signs of hypothyroidism are reversible, once treatment is started.
Cases, Genetics, Breeding
There are many causes of hypothyroidism. One of the most common causes of this disease is the immune system attacking the thyroid gland. This form of the disease is called autoimmune thyroiditis or Hashimoto's Disease. This immune attack of the thyroid ultimately leads to the gland's destruction and the onset of the clinical signs. The reasons that the immune system starts to attach the thyroid gland are poorly understood. However, genetics undoubtedly plays an important role, as this type of hypothyroidism has a high incidence in certain breeds and within certain lines of a breed. It was demonstrated that this type of hypothyroidism is inherited in a polygenetic pattern in beagles. Briards are not listed as a breed that is particularly prone to hypothyroidism. However, when rates of disease prevalence are calculated, often the number of individual animals within a breed are not considered. Thus, it could be that Briards are not listed as being particularly prone to development of hypothyroidism, simply because there are not as many Briards relative to the more popular breeds. We know that hypothyroidism occurs in Briards and that certain lines tend to develop the disease more frequently Thus, until proven otherwise, we should assume that similar inheritance pattern of hypothyroidism occurs in the Briard as well.
So what does this mean from a practical standpoint for the Briard breeder? It is important to point out that not all cases of hypothyroidism are genetically related. There are many other causes, ranging from idiopathic (a fancy work meaning unknown), surgical removal of the gland, cancer, low iodine in the diet (usually only a problem when good commercial dog foods are not being fed) as well as many other causes. Thus, if one puppy develops hypothyroidism it is not necessarily a cause for alarm. However, if two or three of the puppies from a given litter develop the disease or several ancestors in a pup's pedigree had hypothyroidism, then the possibility of a genetic link is very real and care should be taken to assure that this trait is not passed on to future generations. Obviously, a dog that develops this disease should not be bred, unless the breeder is absolutely sure that the hypothyroidism in this dog is not genetically based. When in doubt, err on the side of caution.
How about breeding a normal animal which has a hypothyroid sibling(s) or has substantial hypothyroidism in its pedigree? There is no questions that it would be best not to breed this animal, unless it was truly outstanding. If such an animal is an exceptional specimen and breeding is desire precautions should be taken. One such precaution is that the dog should be tested by the Orthopedic Foundation for Animals for evidence of hypothyroidism. OFA maintains a registry for hypothyroidism, similar to the one maintained for hip dysplasia. They use three tests to evaluate if the animal is or has the potential to become hypothyroid. These tests are:
A problem with these tests is that the dog may be normal at the time of testing, but may develop the condition later. Thus, unlike OFA hip evaluations, normal results does not mean that the dog will never develop the disease. The best these tests can offer is to indicate that the animal does not appear to have the condition at the time of testing. Thus, OFA recommends testing at 2,3,4,6, and 8 years of age. If you breed an animal at three years old, based upon these negative tests, and later discover he tests positive at 6 years of age, then you have passed the potential of hypothyroidism on to the next generation. However, if the tests are positive at three years old, you would know not to breed this animal. So if the thyroid tests are negative and the animal is worth the risk of breeding a potential hypothyroid dog, then outcrossing to a line without any history of hypothyroidism could be attempted, but it is a risk.
Since hypothyroidism is difficult to diagnose, it is debatable whether animals that are of low risk of developing this condition should be registered with the OFA thyroid registry. These tests are costly, need to be frequently repeated, and determining the thyroid function based solely on laboratory findings is not generally a sound diagnostic practice. Certainly conducting these tests will do no harm, but may be of limited benefit to animals that are not genetically predisposed to hypothyroidism.
From this article, one can see that hypothyroidism can be a difficult condition to diagnose. But once diagnosed correctly this condition is usually completely manageable through the use of thyroid supplementation. Through the wise use of selective breeding, we should be able to reduce the incidence of hypothyroidism in the Briard.
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