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Hypothyroidism


Thyroid dysfunction is the most frequently seen endocrine problem in dogs. Virtually all breeds of dogs are affected.

While epidemiological data is scarce, it appears that thyroid disorders occur more frequently in certain breeds and lines, especially in large dogs. This fact indicates a genetic mode of inheritance.

The thyroid gland is involved in regulating the metabolism of all cellular functions. For that reason a reduction of thyroid function leads to a wide range of clinical symptoms. This fact makes it difficult to accurately diagnose this disorder without appropriate laboratory testing and experienced professional interpretation of the results. The most common form of hypothyroidism is autoimmune thyroiditis, which is a familial autoimmune disease of inherited disposition. In this case, the animal's own immune system attacks the thyroid tissue and eventually destroys the thyroid gland. The body will compensate for a period of time by increasing the production of thyroid hormones but once the reserves are depleted the animal will show clinical signs of hypothyroidism.

Thyroid Physiology
The thyroid gland consists of two lobes surrounding the trachea. It has been determined that it produces at least two related hormones thyroxin (T4) and triiodothyronine (T3). The only structural difference is 3 iodine atoms attached to T3 versus 4 atoms attached to T4. About 90% of thyroid hormones secreted are T4 but T3 is considered the active form of the hormone. T4 is converted into the active T3 in peripheral tissues. The conversion pathway is rather complicated and involves series of steps. Both T4 and T3 are initially secreted into the blood and bound to plasma proteins. Only unbound proteins are able to cross into various cells and exert their influence on the synthesis of various metabolic enzymes. Both hormones are not secreted continuously but rather on demand. Once circulating levels of thyroid hormones fall below a certain level, thyroid stimulating hormone (TSH) is produced to stimulate the gland to produce additional thyroid hormones as needed. It also maintains the structural integrity of the thyroid. That is why an early diagnosis and treatment of thyroid disorders in crucial before the gland is permanently damaged.

Signs of Hypothyroidism
Clinical signs of hypothyroidism may include some of all the following: hair loss, seborrhea (oily skin), coarse coat, stunted growth, bacterial skin infections, excessive skin pigmentation, obesity, lethargy, muscular or nerve weakness, exercise intolerance, hyperexcitability, megaesophagus, serious neurological disturbances, unprovoked aggression, seizures, slow heart rate, cardiac arrhythmias, cardiomyopathy (enlarged heart). infertility, lack of libido, decreased or absent sperm count, irregular or absent heat cycles, silent heats, pseudopregnancy, weak, dying or stillborn pups, vomiting, diarrhea, low red and white cell counts, eye problems, loss of smell, chronic active hepatitis to name a few. Table 1 summarizes the symptoms in depth.

Causes of Hypothyroidism
Hypothyroidism is considered to be a genetic disease with an undetermined mode of inheritance in most breeds. So far it has been shown to be inherited as a recessive trait in Borzois. The indications are clear that genetic predisposition for thyroid disorders exists. In addition, certain environmental factors such as nutrition, diet, vaccinations, and exposure to toxins will at the very least trigger the disease in genetically susceptible animals. This is yet another reason to follow a natural approach to the complex issues of nutrition, vaccination schedules, toxin and preservative exposure, and general care with respect to our animal friends (and humans too). It is a well-known fact that the ability of an organism to respond to changes in the environment will greatly depend on its ability to detoxify itself. As the immune system becomes overwhelmed by negative external influences, it will be less competent to maintain an equilibrium. This will manifest itself in many ways: hypothyroidism, allergies, behavior changes being some of them. Some of the triggering factors in an immune system breakdown are genetic predisposition, viral exposure, improper use of modified live vaccines, preservatives, poor diet, stress, and a number of others.

Testing and Diagnosis
There are several different tests available to determine the thyroid function of an individual. The most common tests are total T4 and T3 concentration (TT4 and TT3), free T4 concentration (FT4), TSH concentration, TSH and TRH stimulation tests, thyroglobulin and thyroid hormone antibody levels, and several other tests. None of these tests alone will be sufficient to permit an accurate evaluation of hypothyroidism. A comprehensive thyroid panel together with the interpretation by an experienced clinician will be necessary for the correct diagnosis. Dr. Jean Dodds is one of the leading authorities on canine immune system and hypothyroidism. Her lab provides comprehensive thyroid testing (and other blood tests) at very reasonable prices and her personal interpretation of results and free consultation as well. (W. Jean Dodds, DVM: 714-891-2022 ) It is crucial to have the antibody levels measured as well. Circulating antibodies against thyroglobulin and/or thyroid hormones are seen in cases caused by autoimmune disorder. An even completely healthy animal that possesses abnormally high levels of these antibodies will eventually develop hypothyroidism and its associated clinical signs. These types of cases are likely to be inherited or at least familial.

Genetic Screening
It should be obvious how serious and far-reaching thyroid problems are. It should also be obvious that there is a definite genetic component in the development of thyroid disease. Al breeders must make the decision to screen their breeding stock by utilizing all available means and breed only unaffected individuals to promote the heath and well-being of their respective breeds. All puppy buyers must educate themselves and demand responsible breeding decisions from their breeders.

Thyroid testing should be performed after an animal has reached a sexual maturity (about 10-14 months for males and after the first heat in females). Females must be tested during anestrus to remove any influence of sex hormones. This is usually 12 weeks after the onset of previous heat and lasts for a month. This applies to other health screenings as well. Annual retesting is recommended to compare the results and permit an early diagnosis of developing thyroid disorder.