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Hypothyroidism

OVERVIEW

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 metabolism of all cellular functions. For that resason 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 blood and bound to plasma proteins. Only unbound proteins are able to cross into various cells and exert their influence on 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, hyper excitability, mega esophagus, 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.

There is anecdotal evidence that certain traits tend to be inherited together, which implies a certain degree of linkage. According to German Shepherd Dog breeders, lines that carry the hook tail recessive tend to produce higher than normal percentage of hypothyroid dogs. Some breeders compensate for the possible hypothyroid dysfunction in their breeding stock by medicating all their breeding dogs with thyroid hormone replacement. The horrific damage it causes to their dogs and to the breed in general should be obvious.

CAUSES OF HYPOTHYROIDISM

According to both Drs. George Padgett and Malcolm Willis, hypothyroidism is considered to be a genetic disease with 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 organism to respond to changes in 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 manifests 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 number of others. Some environmental factors influencing the immune system and thyroid function are summarized in Table 2.

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. They are all referenced in Table 3. 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. 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. Even completely healthy animal that possess 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.

TREATMENT

Hypothyroidism is relatively easy to treat. There are several different medications available. Affected animals will lead normal lives. In this respect it is not the most serious canine genetic disease. But the emphasis is on producing healthy dogs, not dogs with easily treatable problems.

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 in 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 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 last 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 disorders.

References

• Padgett, George DVM, Control of Canine Genetic Diseases, Howell Book House, New York, 1998


• Willis, Malcolm B., Practical Genetics for Dog Breeders, Howell Book House,
  New York, 1992


• Wolfsheimer DVM PhD, Brady, Colleen B.A., Thyroid Testing in Dogs: A
  Reference for Dog Breeders and Owners


• Dodds, W. Jean DVM, The Immune System and Disease Resistance


• Dodds, W. Jean DVM, Autoimmune Thyroid Disease: A Common Problem in Purebred Dogs


• The Connection Between Low Thyroid and Seizures: An Interview with W. Jean Dodds, DVM by Joanne Carson, PhD with Comments by W. B. Thomas, DVM


• Dodds, W. Jean DVM, Aronson, Linda P. DVM, Behavioral Changes Associated
  with Thyroid Dysfunction in Dogs


• Dodds, W. Jean DVM, Thyroid Can Alter Behavior


• Dodds, W. Jean DVM, Dr. W. Jean Dodds Non-profit Research Study


• Dodds, W. Jean DVM, Information form to be included with blood sample
  for non-profit research study.


• Cargill, John MA, MBS, MS, Thorpe-Vargas, Susan MS, PhD, Hypothyroidism

TABLE 1

Types of Symptoms	Clinical Signs
Cellular metabolism	Lethargy, mental dullness, exercise intolerance, neurological signs, polyneuropathy (kidney problems), seizures, obesity, cold intolerance, mood swings, hyper excitability, stunted growth, chronic infections
Behavioral changes	Unprovoked aggression towards other animals and/or people, sudden onset of a seizure disorder in adulthood, disorientation, moodiness, erratic temperament, periods of hyperactivity, hypoattentiveness, depression, fearfulness and phobias, anxiety, submissiveness, passivity, compulsiveness, and irritability
Neuromuscular	Weakness, stiffness, laryngeal paralysis, facial paralysis, “tragic expression”, knuckling and/or dragging feet, muscle wasting, mega esophagus, head tilt, drooping eyelids
Dermatological	Dry, scaly skin with dandruff, coarse and dull coat, bilaterally symmetrical hair loss, “rat tail”, “puppy coat”, hyper pigmentation, seborrhea (greasy skin), pyoderma (skin infections), myxedema (skin thickening), chronic offensive skin odor, chronic ear infections, poor wound healing
Reproductive	Infertility, lack of libido, testicular atrophy, hypospermia, aspermia, prolonged interestrus interval, absence of heat cycles, silent heats, false pregnancy, low birth weights, weak, dying, or stillborn pups, excessive lactation, spontaneous abortions
Cardiac	Slow heart rate (bradycardia), arrythmias, cardiomyopathy
Gastrointestinal	Constipation, diarrhea, vomiting
Hematological	Bleeding, bone marrow failure, low red, white, and./or platelet counts, elevated cholesterol levels
Ocular	Corneal lipid deposits, corneal ulceration, uveitis, kerratoconjunctivitis sicca (dry eye), eyelid gland infections (Meibomian gland), Vogt-Koyanagi-Harada syndrome
Other	IgA deficiency, loss of smell, loss of taste, glycosuria (sugar in urine), chronic active hepatitis, other endocrine problems – adrenal, pancreatic, parathyroid

TABLE 2

Common factors associated with autoimmune disease and hypothyroidism
Sex	2:1 females
Genetic or familial history	Increasing frequency
Stress	Environmental, physiological, emotional
Nutritional deficiencies	Selenium – found in vegetables, cereals, meat; Iron – found in vegetables, cereals, meat
Adverse drug reactions	Sulfa-based medications, nitro furan, butazolidin, Phenobarbital, ivermectin, mylbemycin oxime (Interceptor)
Viral infections	Parvovirus, retroviruses, cytomegalovirus, measles and distemper viruses, hepatitis viruses
Use of MLV vaccines	Frequent or recent use of – parvovirus, distemper, hepatitis – Lyme (separately or in combination), Bordetella, rabies
Underlying or concomitant disease	Lymphoma, leukemia, bone marrow failure, chronic infections
Pollutants	Metal processing compounds, plastic and wood processing compounds, rubber processing compounds, mothballs, toilet deodorizers, solvents, plastic, etc.

TABLE 3

Test	Advantages	Limitations
TT3	Inexpensive, simple	Does not reflect thyroid function
TT4	Inexpensive, simple	Does not reflect thyroid function
Free T4	Better indicator of thyroid function	Relatively expensive
TSH concentration	Potentially best indicator of thyroid function, especially when evaluated with free T4	Somewhat expensive
Thyroglobulin antibodies	Early detection of immune mediated thyroid dysfunction	Can be positive for normal dogs
Thyroid hormone antibodies	Early detection of immune mediated thyroid dysfunction	Can be positive for normal dogs

 

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