Thyroid Function Tests By Dr Kirsten Wylie BVSc
The thyroid gland controls the metabolic rate of the body. When the gland functions insufficiently a disease known as hypothyroidism develops.
Dogs that are clinically affected may exhibit one or more of the following clinical signs: Weakness, lethargy, weight gain, skin and coat problems, behavioral changes and infertility. The signs are often vague and not specific.
Canine hypothyroidism is frequently misunderstood and misdiagnosed because there is no single reliable test to make a diagnosis and many diseases affect thyroid function and thyroid tests.
APPROACH TO DIAGNOSING HYPOTHYROIDISM
If your dog has signs suggesting hypothyroidism a full medical work up is needed by your Veterinarian. This includes a full physical check up, complete blood screening (CBC and biochemistry tests) as well as urinalysis. It is vital to inform your Veterinarian of any medications your dog is receiving as many drugs affect thyroid tests. After all this information is obtained to determine if there are complicating medical problems, thyroid tests can be done.
ALL THYROID TEST RESULTS MUST BE INTERPRETED IN LIGHT OF THE BREED, HISTORY AND RESULTS OF A MINIMUM DATA BASE (i.e. the complete blood count, biochemistry panel and urine screen).
THYROID TESTS YOU MAY HAVE READ ABOUT
Total T4 - this test is inexpensive and the best test available in NZ for trying to diagnose hypothyroidism.
- A normal result will nearly always rule out hypothyroidism (10% of hypothyroid dogs have a normal result).
- A low level supports a diagnosis of hypothyroidism
- The Sighthound breeds have lower normal levels.
TSH (Thyroid stimulating hormone)
- must be tested in conjunction with a Total T4.
- Normal results can be seen in 20% of hypothyroid dogs.
- A low T4 and high TSH are diagnostic for hypothyroidism.
- Available through Gribbles Laboratory.
- A normal result rules out hypothyroidism.
- The advantage over a TT4 is that it is less affected by drugs and other diseases, and has a higher sensitivity.
Total T3 and Free T3 - These are of no diagnostic value in trying to diagnose hypothyroidism.
TgAA (Thyroglobulin autoantibody)
There is no one single test that is an accurate indicator of the thyroid status of your animal.
The presence of these autoantibodies indicates an immune mediated process involved in the development of hypothyroidism in some cases. A positive TgAA test is suggestive of future development of the disease as it is an early indicator of problems but to date data has shown that only 20% of dogs testing positive for this actually go on to develop a problem in the near future.
- 50-60% of dogs with hypothyroidism are positive for TgAA.
- When other tests are normal (fT4, T4 and TSH) its predictive value in a given patient remains to be determined.
The thyroid levels must be looked at in conjunction with clinical signs and other laboratory tests.
- normal TT4 or Free T4 and TSH almost always identifies a euthyroid (normal) animal.
- Low TT4 or free T4 and high TSH confirms hypothyroidism.
- Demonstration of a positive TgAA is most valuable in support of an abnormal TT4, Ft4 and/or TSH. When other tests are normal its predictive value in a given patient remains to be determined.
- A low TT3 is probably only of diagnostic value to support the diagnosis of hypothyroidism in sighthounds.
PREDICTING AND PREVENTING HYPOTHRYOIDISM
There is currently no single test that will accurately predict the hereditary status of hypothyroidism.
Recommendations are that breeding dogs from high risk breeds should be screened annually and animals with clinical hypothyroidism should be refrained from breeding from.
Studies on the mode of inheritance of hereditary hypothyroidism/autoimmune thyroiditis in dogs have been inconclusive to date. With the mapping of the canine genome there is hope that they will identify the genes responsible for hereditary hypothyroidism.
What we do know is that some breeds have a greater likelihood of developing hypothyroidism whilst some have a lower prevalence.
High Prevalence (>9% affected) - data from Michigan State University endocrinology laboratory
English Setter, Dalmatian, Basenji, Rhodesian Ridgeback, Old English Sheepdog, Boxer, Maltese, Chesapeake Bay Retriever, Beagle, Cocker Spaniel, Shetland Sheepdog, Siberian Husky, Border Collie, Akita, Golden Retriever.
Low Prevalence (<3% affected) - from Michigan State University endocrinology laboratory.
Chihuahua, Lhasa Apso, Pomeranian, Miniature Pinscher, Cairn Terrier, Basset Hound, Schnauzer, Yorkshire Terrier, Boston Terrier, Norwegian Elkhound, Greyhound, Portugese Water Dog, Newfoundland, Bichon Frise, Pembroke Welsh Corgi, Miniature Schnauzer, Cavalier King Charles Spaniel, Flat Coated Retriever.
Thanks to: Dr Nicki Shackleton BSc, BVSc (Distinction), DACVP (Clinical Pathology) for her help preparing and proof reading the article.
Testing for Hypothyroidism in Dogs
Duncan C Ferguson VMD, PhD, DACVIM, DACVCP Dept Veterinary Biosciences, College of Veterinary Medicine University of Illinois
Vet Clinics Small Animal Practice Volume 37 (2007) 647-669
Thyroid Function Tests in The Dog: SCE Consensus Refined and Defined
ACVIM 2003 for Society of Comparative Endocrinology (SCE)
Duncan Ferguson; Peter Graham; Peter Kintzer; Richard Nelson & David Panciera
Text book of Canine and Feline Endocrinology and Reproduction (Third Edition) Saunders
Edward C Feldman DVM DACVIM, Richard W Nelson DVM DACVIM