Table of Contents

1. What is the Thyroid?
2. Hypothyroidism
3. What causes Hypothyroidism?
4. Clinical Signs and Symptoms
5. Diagnosis
6. Treatment
7. Things that can alter the test
8. Breeding and Genetics
9. Thyroid Disease can lead to Other Problems
10. Registry
11. Links
    
    

    What is the Thyroid?

    I think everyone knows that we have a thyroid gland located somewhere in our neck. Dogs have one as well. In a dog the thyroid gland consists of two lobes in the trachea (or windpipe). It has been established that it secretes at least two related hormones Thyroxin (T4) and Triiodothyronine (T3). The only difference between the two is that T4 has four iodine atoms attached to it and T3 only has three. Even though the thyroid gland secretes mostly T4 (about 90%), it is T3 that is considered the active form of the hormone. In a series of complex steps that involve mainly the liver and kidneys, T4 is stripped of an iodine atom and is converted to T3 when needed. Initially, the two forms are released into the blood and are bound to proteins in the plasma (the clear fluid of the blood). Less than 1% of the T3 and less than .1% of T4 remain in the unbound state in the blood.
    
    

    Hypothyroidism

    Hypothyroidism occurs when the gland stops functioning and producing the thyroid hormone. It is one of the most common hormonal diseases in dogs. While their may be differences in what should be considered normal thyroid levels by breed of dog, it is all too apparent that there are currently many breeds which are more likely to have low thyroid function (inculding the APBT and AST). While epidemiological studies are few and somewhat conflicting, it appears that hypothyroidism occurs more frequently in certain breeds and lines, especially in large breed dogs. 12 breeds most commonly affected were Golden Retriever, Shetland Sheepdog, American Cocker Spaniel, Boxer, Doberman Pinscher, Labrador Retriever, German Shepherd, Akita, Irish Setter, English Setter, Old English Sheepdog, and Collie. Though the APBT and AST are not yet on the �most commonly affected list� it still is often seen in our breed. More than 70% of the 140 breeds recognized by the American Kennel Club (AKC) are genetically predisposed to hypothyroidism. It is not a �life-threatening disease�, but left untreated lethargy will occur, and the dog may experience a mental dullness and the heart rate can slow. Hypothyroidism results from progressive loss of functioning thyroid tissue. The disease can occur in either sex and usually becomes apparent in young adults (1-3 years of age) or middle-aged dogs (4-9 years of age). It is believed that there are five types of hypothyroidism observed in the dog. Of these, three are of the "goiter" type. "Goiter" means that the thyroid gland is enlarged. Enlargement may be due to a reaction to decreased thyroid hormone levels in the blood, there may be a lowered level of iodine in the system (this is usually seen only in dogs fed an all meat diet) or the gland may be affected by tumors, either benign or malignant. All of these situations are rare. The two types of non-goiterous hypothyroidism are primary and acquired. Primary is due to insufficient functioning of thyroid tissue which may mean there is too little tissue available, in short, thyroid atrophy. This situation usually appears in older adults. Why the thyroid should atrophy is not understood, but is considered to be an autoimmune involvement such as the type sometimes observed in Beagles. The last type is linked with pituitary insufficiency. This may occur because the pituitary gland is not secreting normally due to destruction of its parts involved with various types of tumors. In these cases, the pituitary can no longer produce the hormones required by the body, or at least produces them on a reduced basis.
    
    

    What causes Hypothyroidism?

    Hypothyroidism usually happens for unknown reasons. Some believe that Hypothyroidism is caused by:
    *Genetics- it is clear that there is a genetic predisposition for thyroid disorders. The majority of dogs that develop autoantibodies at any time in the dog�s life is an indication that the dog, most likely, has a genetic form of the disease.
    *Immune System- it is believed that a destruction of the thyroid gland can be caused by the immune system. Or that a weak immune system will over work the gland witch will eventually lead to Hypothyroidism.
    *Environmental factors- Exposure to chemical toxins, that at the very least will trigger the onset of disease in the genetically susceptible individual. Diet and Nutrition, the effect of soybeans in the diet can effect the thyroid function. Another factor that needs to be explored is the companies manufacturing animal feeds use toxins to extract the oil from the soybeans. These toxins are lethal. S-(dichlorovinyl) 1-cystine is supposedly not being used anymore, but the chemicals now being used in its place are just as lethal. Residuals of the toxins remain in the feeds and over a period of time the animals will show symptoms of lethargy, lack of interest, lack of appetite, affected hair coat, etc. It is best to avoid soy oil in the diet all together.
    *Vaccinations- Many holistic vets believe thyroid problem is caused by vaccinations.
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    Allergies and other external factors can effect our perception of the prevalence of thyroid disease.
    *Spayed Females- There is a high instance in of hypothyroidism in spayed females. Many believe it is from the change in body hormones after the surgery is preformed. But cause is basically still unknown.
    
    

    Clincial Signs

    Symptoms are usually subtle at first because it is a gradual process that takes months to one year to develop. The symptoms can also be sometimes contradictory and can mimic other diseases. It has also been estimated that approximately 10% of cases with clinical signs actually have the disease. Some clinical signs: Hair loss, itchy skin, odorous skin, dark pigment, overweight (obesity) even when not feed very much, lethargic, weak, less active, seeking out heat, dull coat, skin infections, constipation, diarrhea, cold intolerance, greasy skin, dry skin, reproductive problems, aggression, mental dullness, a decrease in tolerance for exercise, behavioral changes, sudden onset of a seizure disorder in adulthood, disorientation, muscle loss, mental dullness, stunted growth, chronic infections, bleeding, Von Willebrand�s Disease, bone marrow failure, low red blood cell count (anemia), low white blood cell count, low platelet, eye problems, stiffness, paralysis, knuckling or dragging feet, head tilt, cardiac abnormalities (heart problems), vomiting, absence of heat cycles, silent heat, dying or still born pups, loss of smell, loss of taste, active hepatitis and more.
    Symptoms usually appear between one and five years of age, but blood tests indicate the potential for disease before clinical signs appear. Pet owners usually attribute many of these symptoms to the pets aging process. These are all serious problems and tricky to diagnose because the symptoms are also the symptoms of other diseases such as Cushings Syndrome, diabetes mellitus, etc. Therefore, a series of tests should be made to verify what the real cause of the problem might be.
    
    

    Diagnosis

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    Treatment

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    Things that can alter the test

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    Breeding and Genetics

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    Thyroid Disease can cause other problems

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    Registry

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    Links

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    Facts and information in this web page came from Books, Web Sites, and Magazine articles on Thyroid Disease. More information coming soon!
    
    
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