Hypothyroidism

Research has shown that Hypothyroidism is a very common disease in dogs and some breeds have a much higher incidence of the disease than others.  The Basenji is one of these breeds.

Thyroid disease is a pretty complex subject due to the array of medical names given to the various stages and types of thyroid dysfunction plus the many thyroid tests available to measure the hormone levels.  Correct diagnosis and interpretation of test results can also be challenging due to cross reactions, prescribed medicines and non thyroidal illness affecting the hormone levels.  To write about the many types of thyroid dysfunction and tests available here would be a very involved and there are already many good websites dedicated to this subject with information written by experts.  Therefore the information here is to only explain the Hereditary AUTOIMMUNE LYMPHOCYTIC THYROIDITIS which can progress to Hypothyroidism and is the most common form of Thyroid disease.  With a simple blood test breeders have the ability to test their breeding stock for the disease with a highly specific test for Thyroglobulin Autoantibody (TgAA) which detects Autoimmune Lymphocytic Thyroiditis.

Overview
The thyroid gland is considered the Master Gland as any malfunction affects every cell in the body.  In the dog, the thyroid gland consists of two separate lobes which lie either side of the trachea in the neck.   The thyroid's primary function is the production of Thyroxine, a hormone which allows the dog to maintain an adequate and consistent metabolism.  When there is not enough circulating Thyroxine, Hypothyroidism occurs and clinical signs can be seen in the dog.  The most common cause of Hypothyroidism is Lymphocytic Thyroiditis which is a primary disease of the thyroid gland.  At least 50% but some predict up to 95% of Hypothyroidism cases are caused by Lymphocytic Thyroiditis which is an autoimmune disease.  Autoimmune means failure of the immune system to differentiate the body’s own cells from foreign substances, therefore the dogs own lymphocytes (white blood cells) gradually destroy it's thyroid cells which produce, store and secrete thyroxine.  It is estimated that clinical symptoms do not begin to appear in the dog until over 60% of the thyroid gland is destroyed.  Most importantly for dog breeders' studies have established that Autoimmune Lymphocytic Thyroiditis is inherited.

Affected dogs are born with a healthy thyroid gland.  Sub clinical signs detectable by pathology do not usually appear until after puberty.  Thyroid destruction has a gradual onset and can take anything from a few months to years before clinical signs of Hypothyroidism are actually observed in the dog.  Common symptoms are skin/coat changes, allergies, lethargy, weight gain, mental dullness and exercise intolerance.

Previously the only way to positively confirm Autoimmune Lymphocytic Thyroid disease has been by biopsy or Autopsy.  Now diagnosis can be made with a non invasive Thyroglobulin Autoantibody (TgAA) test.  This is a highly specific blood test that reveals if the dog has inherited thyroid disease.  Most affected dogs will have autoantibodies by 4 to 5 years of age.  It is rare for a dog to be positive for TgAA before one year of age so it is recommended to commence testing after puberty and then annually.  After five years of age a dog which has not tested positive for TgAA is unlikely to develop the disease and should remain clear for life.  It should be stressed here the importance of annual testing in the early years because a dog can go from testing negative to positive to negative again and so each test is just a snap shot of that moment in time.  However, if a dog tests positive at any time then it has the hereditary form of the disease even if it later tests negative.  This is possible due to a bout of the disease where the disease does not always progress to full destruction of the gland.  The gland is left compromised with scar tissue but still with enough intact functional cells.  Not all cases will develop into Hypothyroidism but the genetic abnormality still exists in the dog.

Two good reasons to test for TgAA

To reduce Hypothyroidism in the Basenji breed
The only way to reduce the incidence of Hypothyroidism in the Basenji breed is to screen breeding stock to ensure those with hereditary thyroiditis do not produce offspring with the disease.  At this stage it is not known whether hereditary Autoimmune Lymphocytic Thyroiditis is polygenetic or a simple recessive gene therefore breeder's must decide whether to remove an affected dog from the gene pool or breed wisely to a dog negative for TgAA.  This decision may be made on the quality of the individual and other factors such as the importance of continuing an uncommon line to prevent losing it to the gene pool. Can other members of the family be used in breeding who do not carry the gene/s?  

As an aid to early diagnosis of thyroid dysfunction
If a dog tests positive for TgAA and is discovered early from routine testing you will be ready to deal with the onset of the Hypothyroidism.  The dogs Thyroxine levels can be monitored so that treatment can be started at an earlier stage, allowing for better control, with improved prospects of avoiding the chronic effects of Hypothyroidism.  Treatment is inexpensive, relatively easy and very effective. 

How to get your Basenjis tested

Discuss testing with other breeder's in your area and ask your State Basenji Club to organise blood testing days.  This should result in cheaper testing as the more samples tested in a lab run the more cost effective this makes it.

Ask your vet to contact IDEXX Laboratories www.idexx.com.au Telephone 1300 799 722.  IDEXX will supply the sample collection tubes on request.  They will also provide pre printed consignment notes and cover the courier costs for samples submitted to their laboratory.  This is a Toll Priority Overnight Service.  Once your vet has received these items from IDEXX you can arrange an appointment for blood collection.  IDEXX provide your vet with details on how to collect the samples.  The TgAA test is done according to demand and IDEXX require a minimum of three samples before performing a run.  They generally do the test on a Friday with samples run in the morning and results out by close of business same day.  Your vet can request that a run is done on a different day and can also request results to be emailed or faxed through.  Below is information from IDEXX.

CANINE THYROGLOBULIN AUTOANTIBODY (TGAA)
Richard Miller, BVSc, MSc, PhD, MACVSc, DipACVP,  Specialist Veterinary Pathologist

IDEXX is pleased to reoffer testing for canine thyroglobulin autoantibodies. This test was previously performed by IDEXX-CVDL.  It was discontinued in 2001.  Immune mediated thyroiditis is due to a genetic defect in dogs.  During the progression of this autoimmune disease, affected animals produce autoantibodies against their own thyroglobulin.  Immune destruction of the thyroid gland may result in clinical hypothyroidism some months or years later. 

 Screening For Breeding Suitability
Some breeds of dog are over represented when tested for the presence of TgAA.  In both sexes of these breeds it is recommended that TgAA be used as a screening test for breeding suitability.  All breeding animals should be checked annually from 1 year until they are 6 years of age.  Animals that are positive for TgAA should not be used for breeding.

Reporting of Results
Canine thyroglobulin autoantibody (TgAA) is reported as a %. The result compares the absorbance of the ELISA test result against the test positive control.

Interpretation of Results
Results of greater than 35% are positive for TgAA
Results of 20-35% are inconclusive for TgAA
Results of less than 20% are negative for TgAA

General Interpretative Guidelines
Positive results indicate the presence of autoantibodies against canine thyroglobulin. This may or may not be associated with clinical hypothyroidism. Canine T4 or free T4 by equilibrium dialysis should be measured to help interpret the TgAA result.  An animal less than 5 years of age with positive TgAA and normal T4 should be monitored for the eventual development of hypothyroidism by measuring T4 or free T4 on an annual basis.

For more reading on this subject please follow these links

References: Orthopaedic Foundation for Animals, Michigan State University, Oxford Labs, Idexx Labs

Copyright © Davina Hopkins 2005