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

This book is a must for all breeders. It deals in details with canine genetics and ways to identify and eliminate genetic diseases.

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

Another good book on genetics.

ONLINE REFERENCES

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

http://www.golden-retriever.com/dodds.html

http://canine-epilepsy-guardian-angels.com/ImmuneSystem.htm

An incredible resource about immune system and associated health issues. It touches on immune system and how it works, hypothyroidism, autoimmune disease in general, viruses, cancer, vaccines, nutritional factors, etc. It includes 2 comprehensive tables that list factors associated with autoimmune disorders and clinical signs of hypothyroidism. Very highly recommended.

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

http://canine-epilepsy-guardian-angels.com/ThyroidDisease.htm

This article discusses specific recommendations and intervals for thyroid health screening, type and frequency of treatment, as well as clinical signs of hypothyroidism.

Carson, Joanne PhD, Thomas, W.B. DVM, Dodds, W. Jean DVM, 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

http://canine-epilepsy-guardian-angels.com/InterviewDrDoddsAndJoanneCarson.htm

An overview of hypothyroidism. An interesting article in a format of Q&A.

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

http://canine-epilepsy-guardian-angels.com/behaviorandthyroid.htm

Published Study: 77% of dogs with seizures are Hypothyroid. This article also describes behavioral changes common in hypothyroid dogs, such as 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

Dodds, W. Jean DVM, Thyroid Can Alter Behavior

http://canine-epilepsy-guardian-angels.com/bizarre_behavior.htm

As published in Dog World Vol. 77 No. 10, October 1992, this article describes possible connection between bizarre behavior and hypothyroidism.

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

http://canine-epilepsy-guardian-angels.com/DrDoddsInstructions.htm

This article includes specific instructions for several tests including hypothyroidism test. It had Dr. Dodds contact info. She is very willing to help or answer questions – personal observation.

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

http://canine-epilepsy-guardian-angels.com/DrDoddsInfoForm.htm

Self explanatory, test order form. Includes current prices for certain tests. Other tests are available as well at excellent prices – call the lab.

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

http://acmepet.petsmart.com/content/health/thyroid.html

http://siriusdog.com/hypothyroidism_clinical.htm

Two links to the same excellent article that describes the thyroid gland function, testing, treatment, etc. in easy to understand terms. Authors take a hard line approach to genetic testing for hypothyroidism – do it or do not breed. Includes several figures that detail clinical signs, causes, testing protocols, normal testing ranges and affected breeds.

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

http://www.golden-retriever.com/thyr-lsu.html

Very detailed article about thyroid testing.

Zarini, Nilda, Thyroid

http://members.home.net/zara/hypothyroid.htm

Very informative and comprehensive article.

Autoimmune Disease

http://sirius.com/autoimmune.htm

Excellent article that discussed the immune system, environmental influences, several autoimmune diseases - Autoimmune hemolytic anemia, Immune-mediated thrombocytopenia, Autoimmune diseases of the skin (Pemphigus complex, Pemphigus vulgaris, Pemphigus vegetans, Pemphigus foliaceous, Pemphigus erythematosis, bullous pemphigoid, Discoid lupus erythematosis), Vogt-Koyanagi-Harada-like syndrome (VKH), Systemic lupus erythematosis, polymyositis, glomerulonephritis, polyarthritis, rheumatoid arthritis (RA), polyarthropathies, hypothyroidism, diabetes mellitus, kerratoconjunctivitis sicca (KCS or “dry eye”), chronic active hepatitis (liver disease. It includes symptoms, diagnosis, and treatment.

http://thyroid.miningco.com/health/thyroid/msub31.htm

This web site has numerous links to articles or resources about various subtopics on hypothyroidism.

Notes in italics are the author’s personal comments.

This article can be freely reprinted as long as credits are given to the author. Comments or suggestions are welcome. Thank you.

Michelle Pelescak

mpshiloh@hotmail.com

http://www.shilohshepherd-nyssa.com
Copyright © 2000 MD Shilo Star Kennel. All rights reserved.
Revised:

August 6, 2000

TABLE 1

Types of Symptoms

Clinical Signs

Cellular metabolism

Behavioral changes

Neuromuscular

Dermatological

Reproductive

Cardiac

Gastrointestinal

Hematological

Ocular

Other

Lethargy, mental dullness, exercise intolerance, neurological signs, polyneuropathy (kidney problems), seizures, obesity, cold intolerance, mood swings, hyper excitability, stunted growth, chronic infections

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

Weakness, stiffness, laryngeal paralysis, facial paralysis, “tragic expression”, knuckling and/or dragging feet, muscle wasting, mega esophagus, head tilt, drooping eyelids

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

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

Slow heart rate (bradycardia), arrythmias, cardiomyopathy

Constipation, diarrhea, vomiting

Bleeding, bone marrow failure, low red, white, and./or platelet counts, elevated cholesterol levels

Corneal lipid deposits, corneal ulceration, uveitis, kerratoconjunctivitis sicca (dry eye), eyelid gland infections (Meibomian gland), Vogt-Koyanagi-Harada syndrome

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

Genetic or familial history

Stress

Nutritional deficiencies

Adverse drug reactions

Viral infections

Use of MLV vaccines

Underlying or concomitant disease

Pollutants

2:1 females

Increasing frequency

Environmental, physiological, emotional

Selenium – found in vegetables, cereals, meat

Iron – found in vegetables, cereals, meat

Sulfa-based medications, nitro furan, butazolidin, Phenobarbital, ivermectin, mylbemycin oxime (Interceptor)

Parvovirus, retroviruses, cytomegalovirus, measles and distemper viruses, hepatitis viruses

Frequent or recent use of – parvovirus, distemper, hepatitis – Lyme (separately or in combination), Bordetella, rabies

Lymphoma, leukemia, bone marrow failure, chronic infections

Metal processing compounds, plastic and wood processing compounds, rubber processing compounds, mothballs, toilet deodorizers, solvents, plastic, etc.

TABLE 3

Test

Advantages

Limitations

TT3

TT4

Free T4

TSH concentration

Thyroglobulin antibodies

Thyroid hormone antibodies

Inexpensive, simple

Better indicator of thyroid function

Potentially best indicator of thyroid function, especially when evaluated with free T4

Early detection of immune mediated thyroid dysfunction

Does not reflect thyroid function

Relatively expensive

Somewhat expensive

Can be positive for normal dogs