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HYPOTHYROIDISM IN THE DOG
(This page is being updated the week of 8/7/06)

Hypothyroidism in the dog can be due to a number of issues associated with the production of specific thyroid hormones, with the delivery of those hormones to target tissues, with immune mediated interference with proper chemical structure the hormones and with damage to the thyroid gland itself. Hypothyroidism is the most common endocrine gland disorder in the dog.  The endocrine glands are listed below.  All of these tissues secrete chemical substances called hormones that have a profound effect on certain "target" tissues.  A hormone is a chemical that acts on or with tissues throughout the body and is carried away from the gland of production by the blood stream.  Saliva, for example, is secreted by various salivary glands but is not considered a hormone because it leaves the glands via a small duct (a small tube) and is not transported by the blood stream and does not have an effect on any body tissue physiology.

Of all those glands and hormones, the thyroid gland is considered the Master Gland.  If the thyroid gland is malfunctioning, every cell in the body is affected.  No wonder dogs, properly diagnosed with hypothyroidism and placed on appropriate thyroid hormone supplements, will look, act and feel so much better than before treatment.

SIGNS
Most dogs show signs of hypothyroidism between 3 and 5 years of age but a diagnosis isn't made in some dogs until they are years older.  Males and females seem to be equally affected.  Routine testing of young dogs is not commonly done unless the veterinarian has a reason to suspect low thyroid function.  Some veterinarians believe that if a dog has not acquired hypothyroidism by 5 or 6 years of age, the odds are good that it will never be a problem for that individual.  (Cats rarely acquire hypothyroidism but have more trouble with hyperthyroidism, especially older cats.)  

One of the most commonly seen signs that a dog may have insufficient secretion of thyroid hormone, or issues with immune mediated thyroid dysfunction, is weight gain with apparently little food intake.  Any overweight pet should be checked for hypothyroidism.  (See this page for tips on how to get a pet to lose weight.) In long-standing cases there often is lack of proper hair coat and even hair loss.  This loss generally displays a pattern over the lumbar area on both sides, sparse hairs on the back of the rear legs and lack of hair along the abdomen.  The coat tends to lack luster and the finer hairs of the undercoat may be missing entirely.  Most dogs with hypothyroidism lack energy, prefer warm environments and have poor exercise tolerance. Of great concern to breeders is the fact that dogs with hypothyroidism may be infertile and many breeders have their dogs tested for thyroid function prior to breeding; in dogs with a poor breeding history, hypothyroidism often is the culprit.

DIAGNOSIS
If the physical exam or history indicates probable hypothyroidism, the veterinarian will take a blood sample and have one or more tests run. The most common tests for thyroid function are T4 (the main Thyroid hormone) and canine TSH (Thyroid Stimulating Hormone from the Pituitary Gland). If the screening test using the T4 analysis indicates low a value for T4, further evaluation is very important in determining the actual origin of the dysfunction causing the signs of hypothyroidism.  Some veterinary laboratories now recommend TgAA (Thyroglobin Auto-antibody) analysis be done because it identifies thyroiditis much earlier in the progression of the disease. All of these assays are relatively inexpensive, and the information they provide is very important for establishing a diagnosis.  University Veterinary Medical Colleges such as the U. of Illinois and Michigan State University have state of the art diagnostic instrumentation that have advanced the ability and accuracy of local veterinarians attempts to establish the presence of hypothyroidism in their patients. 

TREATMENT
L-thyroxine (T4) tablets are generally administered twice a day.  When given twice a day, some veterinarians prescribe 0.1 mg/10 pounds twice per day as the initial dosage but each case is considered individually.  Repeat exams and occasional follow-up blood testing really helps to fine tune the proper amount of medication needed for each patient.

Is hypothyroidism inherited?  Evidence indicates some familial patterns of inheritance.  There are more than half a dozen studies reporting the familial incidence of autoimmune thyroiditis where the dog's immune system actually damages its own thyroid gland.  Much of the research on hypothyroidism is dogs has been done at Michigan State University Veterinary School.

Not all cases of hypothyroidism are due to autoimmune lymphocytic infiltration of the gland.  There can be other "inducers" of the disease such as consumption of too much Iodine.  These inducers can be very difficult to identify.  Humans may acquire what is called Hashimoto's Disease, a genetically transmitted form of hypothyroidism but this disease is not the same as autoimmune thyroiditis in dogs.  In Hashimoto's Disease females are five times more likely to get the disease than males.  There are other differences, as well.

CANINE HYPOTHYROIDISM: CASE PRESENTATION

The following case represents a fairly atypical case of hypothyroidism in that most cases are identified prior to such advanced skin and coat signs.  The dog had been treated for non-specific allergies, and in fact does some allergic problems and skin scarring due to chronic inflammation of the skin.  However, once the Hypothyroidism was discovered and proper treatment instituted, the allergic conditions were less severe simply because the skin became much healthier and more resistant to infections, irritants and allergens.

This is how a dog recently looked when presented...  Hypothyroidism was suspected and the blood values of thyroid hormone supported the presumptive diagnosis.   The dog was placed on Soloxine in early June, 2001 and the photos in the second row show the improvement in her after six weeks on medication. You can see what a change can occur when a diagnosis of Hypothyroidism is made and proper therapy instituted. The dog's owners report excellent new growth of hair, renewed vigor and weight loss... all improvements!

See blood sample values below:
Thyroid Gland Evaluation:  Blood Test Values In A Dog

Click on the image to see a close-up view.
Above... Before Treatment                      Below... After Treatment
	Click to enlarge the photos

This dog will need to be on thyroid supplementation for life but the medication is not expensive and the patient has resumed a more normal attitude and looks much improved.  Be sure to ask your veterinarian to check for hypothyroidism if your dog displays a poor coat, weight gain, poor exercise tolerance and seeks warm areas in which to spend time.

(See this page for tips on how to get a pet to lose weight.)

Fall 2005 Information on Greyhounds and hypothyroidism from the Greyhound Club of America website...

THYROID STUDY COMPLETED
Helen Hamilton, DVM
Health Committee Chair

Project: Establishing Thyroid Normals in the Greyhound breed
Researcher:  Dr. Ray Nachinder – Michigan State University,

The study of Greyhounds and Thyroid Testing has been competed and the results reported in an article written by Dr. Marcus Rick and published in the GCA Fall 2005 Newsletter.  Of one hundred samples that were collected, 98 were analyzed statistically to establish a reference range of thyroid test results in the Greyhound.  The majority of the samples were from AKC registered Greyhounds, seven were from NGA dogs.  The results were then compared graphically with the reference range for the general dog population used at Michigan State University and the result ranges from three previous studies by other researchers.

Dr. Rick states “Overall it can be seen that the reference range of Greyhounds is considerably less than that of the general canine reference range.”  In very short summary he concludes:

1.)  Overall, hypothyroidism is very unlikely in the Greyhound, however well documented cases exist.

2.)  If either total T4 of free T4 by dialysis are detectable with the currently available assays (GT 6nmol/L, GT 2pmol/L, respectively) it is very likely that your Greyhound is NOT hypothyroid.

3.)  Lymphocytic thyroiditis is unlikely as well.  Of 1,409 Greyhound samples received at Michigan State University, 2 % were TgAA positive compared with 10 % of 143,800 dogs of the general dog population.  Of these 1,409, only 43 (3%) had thyroid results including elevated TSH consistent with hypothyroidism compared to 8 % of the general 143,800 samples.

4.)  Diagnosing hypothyroidism might be difficult due to decreased reference values that, in many cases, are below the detection limit of currently available assays.

5.)  Greyhound-specific complaints, including bald thigh syndrome, poor racing performance, and infertility do not have an association to thyroid hormone concentrations.  Supplementing these animals with thyroxine is not recommended.