Hypothyroidism

The thyroid gland is a small gland located under the skin at the center of the neck,.  It has two lobes and can be felt with careful palpation.  

The thyroid gland converts iodine into two hormones, thyroxine (T4) and triiodothyronine (T3) and then circulates these hormones through the bloodstream.  The hormones produced by the thyroid gland directly and/or indirectly affect the physiological function of virtually all cells of the body.   It plays a major role in controlling metabolism and is needed for growth.

To control the level of these hormones, the hypothalamus and pituitary glands secrete compounds called releasing factors.  In the case of the thyroid gland, they secrete a releasing factor called thyroid stimulating hormone (TSH).  It's the amount of TSH circulating in the blood stream that tells the thyroid gland how much thyroxine (T4) to secrete.

In the dog, the biologically active hormone is T3 (triiodothyronine).  50% of the circulating T3 comes directly from the thyroid gland.  The other 50% is produced by converting T4 to T3 in tissues and organs such as muscle, kidney, brain, pituitary gland and liver.

Hypothyroidism:

Hypothyroidsim occurs when circulating concentrations of T4 and/or T3 are decreased due to reduced function of the thyroid gland.  Lymphocytic thyroiditis (autoimmune thyroiditis) is the most common cause of primary hypothyroidism in dogs.  The predisposition to lymphocytic thyroiditis is believed to be highly inherited.

With lymphocytic thyroiditis, antibodies progressively attack the thyroid gland which eventually leads to a failure of thyroid hormone production.  More than 60% of the thyroid gland needs to be destroyed before changes in laboratory values are seen.  This process can take months or years.

Symptoms of Hypothyroidism

As stated earlier the thyroid gland can affect every cell in the body, therefore the symptoms are very broad and can mimic other diseases.  Classic symptoms include mental dullness, lethargy, obesity and heat seeking behavior, although many hypothyroid dogs don't have any of these symptoms.  Other symptoms include:

Hair loss

Dull dry coat

Inflamed skin due to secondary infection

Ear infections

Mental dullness

Muscle wasting on the head

Facial paralysis

Disorientation

Muscle weakness, wasting or stiffness

Delayed reflexes

Hyperlipidemia

Cardiac disorders (decreased heart rate and cardiac output)

Immunologic disorders (Recurrent Infections)

Anemia

Gastrointestinal disorders (diarrhea or constipation)

Reproductive disorders

Seizures (see note below)

 

Diagnosing Thyroid Disease

The diagnosis for thyroid disease is based on symptoms and a panel of thyroid function tests.   There are 8-parameters that should be considered in a full thyroid profile.  Most of these parameters are testing the two thyroid hormones T4 and T3 in various different forms.

Total T4 - As indicated above T4 is derived entirely from direct thyroid secretion and therefore would be expected to be an accurate measure of thyroid function.  Unfortunately other factors such as serum binding proteins, age, sex, body, size and breed make total T4 measurements less useful than expected.  Measuring Total T4 alone may lead to a mistaken diagnosis of hypothyroidism.

Free T4 - is the small fraction of Total T4 that is not bound to proteins in the blood.  Only the unbound fraction of T4 (Free T4) is available for interaction with target cells and is believed to more accurately reflect the clinical condition of the thyroid gland.

Total T3 - is useful as part of a thyroid profile but not reliable as a single test.  Because 50% of circulating T3 is produced by conversion of T4 to T3, serum concentrations may be affected by the organs used in the conversion as well as the function of the thyroid gland. 

Free T3 is the biologically active portion of Total T3 and is affected in the same way that Total T3 is.

Auto Antibodies - T3 Auto Antibodies (T3AA) and T4 Auto Antibodies (T4AA) are subsets of a TgAA test.  The presence of auto antibodies are markers for lymphocytic thyroiditis.  T3AA cause a false decrease in Total T3 and a false increase in FT3.  T4AA falsely increases both Total T4 and Free T4 so they are critical in accurately evaluating other thyroid parameters.

TgAA (thyroglobulin autoantibodies) elevations confirm lymphocytic thyroiditis and promotes early detection of the disorder.  TgAA can also facilitate genetic counseling.

TSH (thyroid stimulating hormone) TSH is produced by the pituitary in response to low serum T4 levels.  An increase in TSH concentration, therefore indicates a failure of adequate thyroid hormone production and is valuable as one of the parameters in a thyroid test.

Drug therapy with Phenobarbital or glucocorticoids can lower thyroid hormone concentrations making the diagnosis of hypothyroidism even more difficult.  A careful history, physical examination and laboratory screening tests are all necessary to accurately diagnose hypothyroidism in the dog.

Note:

Although some authors list seizures as a sign of canine hypothyroidism, a true cause and effect relationship has not been proven.  There are however, many case studies that indicate that there may be a relationship and some veterinarians believe that hypothyroidism can exacerbate seizures in susceptible dogs.  Hypothyroidism is common and thyroid testing carries very little risk, therefore most veterinarians would suggest that a thyroid test be a part of a diagnostic work up for any dog with seizures.

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Last Updated August 2009