CANINE HYPOTHYROIDISM

Hypothyroidism in The Dog
What is Hypothyroidism

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.

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!

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

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.

What is  hypothyroidism? http://www.upei.ca/cidd related terms: familial thyroiditis, lymphocytic thyroiditis, congenital hypothyroid dwarfism

The clinical signs of hypothyroidism are caused by a decrease in normal thyroid hormone activity. The disorder may be acquired (a progressive deficiency of thyroid hormone) or congenital (meaning the animal is born with the disorder). The acquired form is the most common disorder of the endocrine system in dogs. It occurs as a result of gradual atrophy of the thyroid gland or of gradual infiltration and replacement of the thyroid gland with lymphocytes due to an autoimmune process (lymphocytic thyroiditis).

Acquired hypothyroidism is generally seen in middle-aged (4 to 10 years) mid - to large breed dogs. Congenital hypothyroidism  is very rare.

How is hypothyroidism inherited?
unknown

What breeds are affected by hypothyroidism?
The following breeds have an increased risk of developing hypothyroidism: Afghan hound, Airedale terrier, boxer, Chinese shar pei, chow chow, cocker spaniel, dachshund, Doberman pinscher, English bulldog, golden retriever, Great Dane, Irish setter, and miniature schnauzer.

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a general consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does hypothyroidism mean to your dog & you?
The changes due to gradually decreasing levels of circulating thyroid hormone are slow and insidious. Early signs (which are usually not recognized as being related to hypothyroidism) include lower energy levels and increased susceptibility to infections. As the disease progresses, you will likely notice changes in your dog's hair coat - symmetrical hair loss with or without darkening of the skin, and dry or greasy hair. Other signs of hypothyroidism include a slow heart rate, lethargy, mental dullness, intolerance to cold, infertility in males and females, constipation, and weight gain. Less commonly, a dog with hypothyroidism may experience heart disease, a bleeding disorder, profound muscular weakness associated with abnormalities in the muscles or nerves, or another endocrine disorder such as diabetes mellitus.

Congenital hypothyroidism
Thyroid hormones are essential for normal growth and maturation of the nervous and skeletal systems. Puppies with congenital hypothyroidism will have stunted growth as well as many other abnormalities. Severely affected puppies most likely die before weaning.

How is hypothyroidism diagnosed?
Because there is such a broad range of possible clinical signs, hypothyroidism can be quite difficult to diagnose. Blood tests may show certain suggestive (but non-specific) abnormalities. If your veterinarian suspects this condition, s/he will perform laboratory tests to assess thyroid function.

FOR THE VETERINARIAN:
CLINICAL PATHOLOGY: Suggestive findings include hypercholesteremia, mild nonregenerative anemia, elevated serum creatine kinase (CK), and hypoglycemia

CONFIRMATION: Measurement of free T4 (FT4) and canine thyroid-stimulating hormone (cTSH) are the tests of choice to diagnose hypothyroidism, and to differentiate the primary from the secondary form.

 [Secondary hypothyroidism is much less common.] Some laboratories can also test for thyroid autoantibody levels (TgAA) which is helpful in the early diagnosis of autoimmune thyroiditis. This may provide useful information for breeders. [See reference 2 for a good discussion of tests.]

RADIOGRAPHS: In congenital hypothyroidism, there are typical radiographic changes including epiphyseal closure, shortened vertebral bodies, kyphosis, and arthritis. Epiphyseal dysgenesis (ragged epiphyses with a few foci of calcification) is pathognomonic for congenital hypothyroidism.

How is hypothyroidism treated?
The standard treatment is levo-thyroxine given once a day. This must be continued for life. Within a week of starting treatment, your dog's attitude and activity levels should improve. It can take up to 6 weeks before there is noticeable improvement in the skin and haircoat, but eventually all abnormalities should completely resolve.

Dogs on thyroid replacement can live a normal life. Your veterinarian will monitor thyroid hormone levels periodically and adjust the dose if required. Certain other illnesses may also necessitate dosage adjustment.

Congenital hypothyroidism is treated the same way. Clinical signs are usually completely reversed with subsequent normal physical and mental growth and development, as long as the condition is recognized and treatment started very early (3 to 4 months of age).

Genetic counselling
Although inheritance of this disorder has not been determined, it is advisable not to breed affected dogs. The OFA (Orthopedic Foundation for Animals) maintains a thyroid registry based on assessment of FT4, cTSH, and TgAA, which is useful for breeders attempting to choose dogs free of hypothyroidism for a breeding programme.

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources
Chastain, C.B., Panciera, D.L. 1995. Hypothyroid diseases. In S.J. Ettinger and E.C. Feldman (eds.) Textbook of veterinary Internal medicine. p. 1487-1501. W.B. Saunders Co., Toronto.
Peterson ME, Melian C, Nichols R. Measurement of serum total thyroxine, triodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. JAVMA  1997; 211(11):1396-1402. This reference clearly explains the different tests available, and the significance and interpretation of the results.
Orthopedic Foundation for Animals: www.offa.org

Copyright © 1998 Canine Inherited Disorders Database. All rights reserved.Revised: October 30, 2001.
reprinted with kind permission from:- Alice Crook, DVM,Coordinator,
 Sir James Dunn Animal Welfare Centre, Atlantic Veterinary College,University of Prince Edward Island, 550 University Ave.Charlottetown, PEI C1A 4P3

http://www.upei.ca/cidd

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Thyroid Testing in Dogs	Phenobarbitol and Thyroid Testing	Hypothyroidism Supplements	Diagnostic Evaluation of The Hypothalamic-Pituitary-Thyroid Axis
T4 Diagnostic Protocol for Dogs	Behavioural Changes Associated with Thyroid Dysfunction in Dogs