FAQs or Common Misconceptions about the Canine Thyroid

As we reflect on Hemopet’s Canine Thyroid Awareness Month from last month, we realized we frequently receive questions throughout the year regarding: signs of canine hypothyroid disorder; canine thyroid testing; and, medication (mostly adjusting it to treat canine hypothyroidism). 

We decided that a refresher course is in order.

While canine and human hypothyroidism are prevalent in both species, diagnosing and treatment are different. So, you need to put aside your knowledge about people based upon going to your own doctor. 

In this blog post, we are modifying and updating some excerpts from my book with Diana Laverdure-Dunetz’s, The Canine Thyroid Epidemic: Answers You Need for Your Dog. If you are interested in lengthier and more contextual explanations, you can pick up the book on Amazon. 

Common Signs of Canine Thyroid Disorder

Many symptoms can be attributed to hypothyroidism in companion dogs. We have picked out the more common universal ones, but provide a more complete list in our book. Ruling out hypothyroidism – or even ruling it in – with proper diagnostic testing can help pinpoint other disorders more accurately or move you along to testing for those as well. Canine thyroid testing is a good first line metabolic test before pursuing more extensive diagnostic testing options. 

  1. Fatigue
  2. Weight gain or obesity – Obesity or overweightness in companion pets is probably the biggest health crisis at the moment. We have a post about how to help your companion dog lose weight called “Putting It into Action: Companion Pet Caloric Needs for Ideal Target Weight.” If this does not resolve your companion dog’s weight problem, your dog is slow to lose the pounds, or other hypothyroid symptoms persist, we strongly suggest you have your companion dog thoroughly tested for hypothyroidism. 
  3. Aggressive Behavior
  4. Cold Intolerance
  5. Exercise Intolerance

Proper Canine Thyroid Testing

Dr. Dodds can interpret your companion dog’s thyroid test results for a small consultation fee. (She honors collegial reciprocity to fellow veterinarians to help advance knowledge, and provide uniformity to, the industry.) 

Unfortunately, it can be frustrating for a companion pet parent when more complete information is needed and a follow-up comprehensive thyroid test is required, which can be run at either Hemopet’s Hemolife Diagnostic Laboratory, Michigan State University or any other veterinary reference laboratory. 

What we prefer is Hemopet’s Thyroid Profile 5™.

The Thyroid Profile 5 includes T4, freeT4, T3, freeT3 and TGAA (Thyroglobulin Autoantibodies). (By the way, since there is an 8% chance of having a false negative TGAA, Hemolife will also perform either the T3AA or T4AA when results warrant it or in cases where T3AA or T4AA were previously done and needed for a follow-up.)

Addressing Common Companion Dog Guardian Questions Regarding Thyroid Testing

The first two items companion dog parents raise with us is their dog’s thyroid stimulating hormone (TSH) and thyroxine (T4).

#1. Let’s address TSH first.

In primary hypothyroidism, as freeT4 levels fall, pituitary output of TSH rises. About 95 percent of thyroid hormone regulation in humans is controlled by TSH, it is a highly accurate screening for hypothyroidism. However, only about 70 percent of thyroid hormone regulation in dogs is controlled by TSH, so this test shows relatively poor predictability. The remaining 30 percent of a dog’s thyroid regulation is controlled by growth hormone, which, like TSH, is manufactured, stored, and secreted by the pituitary gland. For this reason, the TSH test can provide a false negative or false positive result in approximately 30 percent of canine cases. So, although elevated TSH usually indicates primary thyroid disease, there is 20 – 40 percent discordance observed between expected and actual results in normal dogs as well as in hypothyroid dogs or those with non-thyroidal illness (NTI).

#2. Issues with only testing T4.

Many veterinarians believe that serum T4 alone is adequate as the first screening for a thyroid problem, and that only if T4 is abnormal should further testing be pursued. This misconception, which is still perpetuated today by some academics at veterinary schools throughout the country, is a huge obstacle to accurately and efficiently diagnosing canine thyroid disorders. Additionally, current veterinary medical textbooks have stated that if a dog has a T4 level above 2 ug/dl [26 nmol/L), there is no need to perform other thyroid testing because the dog is euthyroid [has a normal thyroid]. This statement is misleading, as the T4 result fails to identify any cases of thyroiditis in which elevated thyroid autoantibodies are present! 

The only way that true progress can be made in the diagnosis of canine thyroid disorder is when veterinarians realize that serum T4 alone is not a reliable method of initial screening, as there are many circumstances in which it can provide misleading results. T4 can over diagnose hypothyroidism in the presence of NTI (such as chronic yeast infections, liver and bowel disorders, and kidney disease – just a few of many examples) or with the use of certain drugs (corticosteroids, phenobarbital, and sulfonamides); it inaccurately assesses the adequacy of thyroxine therapy; and it fails to detect autoimmune thyroiditis.

Veterinarians should not use T4 as the first screening test for hypothyroidism. If the T4 is low, you will not know whether the values are accurate without performing additional tests. If it’s normal you may miss the diagnosis altogether, because there could be a thyroid antibody preventing you from even determining it. 

#3. Thyroid autoantibody testing TGAA?

Canine Thyroglobulin Autoantibodies (TgAA) – Elevated thyroglobulin autoantibodies are present in the serum of dogs with autoimmune thyroiditis, which of course is the heritable form of hypothyroidism. 90 percent of cases of canine hypothyroidism result from the heritable condition. TgAA is especially important in screening breeding stock for autoimmune thyroiditis, as dogs testing positive for TgAA should not be bred. The commercial TgAA test can give false negative results if the dog has received thyroid supplement within the previous 90 days, thereby allowing unscrupulous owners to test dogs while on treatment to assert there normalcy, or to obtain certification with health registries such as the OFA Thyroid Registry. False negative TgAA results can also occur in about eight percent of dogs verified to have high T3AA and/or T4AA. Furthermore, low-grade false positive TgAA results may be obtained if the dog has been vaccinated within the previous 30 – 45 days for rabies, or very occasionally in cases of NTI. 

T3 Autoantibody (T3AA)/T4 Autoantibody (T4AA) – These autoantibodies affect the ability to measure T4 and T3 accurately with most thyroid assay methods. In the presence of high levels of circulating T3AA and/or T4AA, the autoantibody interferes with the ability of the test antibody reagent [reacting substance] to detect the hormone being measured. The result is a spuriously high reading of T3 and freeT3 or T4 and freeT4. However, if the freeT4 is measured by the equilibrium dialysis (ED) technique, the T4AA will be removed by the dialysis step and not be detected. Thus, the presence of T4AA may go unnoticed if freeT4 is only measured by the ED method. Fortunately, most circulating antibodies are against T3 (~70%), some affect both T3 and T4 (~25%), and only a few affect T4 alone (~5%). 

Most cases of autoimmune thyroiditis exhibit elevated serum TgAA levels, whereas only about 20 – 40 percent of cases have elevated circulating T3 and/or T4AA. Thus, the presence of elevated T3 and/or T4AA confirms a diagnosis of autoimmune thyroiditis but underestimates its prevalence, because when these circulating autoantibody levels are normal/negative (non-elevated), it does not rule out thyroiditis. 

#4. Detailing the rest of the canine thyroid testing profile. 

Total T4 – This test measures the total amount of T4 hormone circulating in the blood – both bound and unbound molecules. More than 99 percent of T4 hormone is “bound,” meaning that it attaches to proteins in the blood and never reaches the tissues. Therefore, a T4 result by itself is often misleading, since it is affected by anything that changes the amount of binding proteins circulating in the blood, such as occurs with certain drugs. Unfortunately. T4 is still the most popular and widely used initial screening test for thyroid disorder in dogs. Relying on the accuracy and sensitivity of this test alone is at the heart of the common misdiagnosis of canine thyroid disorder. T4 alone is not an accurate indicator of thyroid disorder in dogs, and is often affected by moderate to severe non-thyroidal illness (NTI) [a disease process other than thyroid disease] and certain medications (e.g. phenobarbital, corticosteroids, and sulfonamides).

Free T4 – Serum freeT4 represents the tiny fraction (< 0.1%) of thyroxine hormone that is unbound and therefore is biologically active. As the freeT4 molecule circulates in the blood and through the pituitary gland’s sensor, the level of free T4 tells the pituitary gland whether or not it needs to make more TSH. Although both the bound and free forms of T4 hormone are in circulation, the pituitary gland only recognizes the free molecule. Since protein levels in the blood do not (or only minimally) affect freeT4, it is considered a more accurate test of true thyroid activity than the total T4. FreeT4 is much less likely to be influenced by NTI or drugs. Both total T4 and free T4 are lowered in cases of true hypothyroidism. While endocrinologists may favor the ED RIA method for measuring freeT4 because earlier analog methods were less accurate, newer technologies (improved analog RIAs and non-RIA chemiluminescence and other methods) offer alternative and accurate methodology. These new assays are also faster and less costly.

Total T3 – As with total T4, total T3 represents both the bound and unbound forms of T3 circulating in the blood. Measuring serum T3 alone is not considered an accurate method of diagnosing canine thyroid disorder, as this hormone reflects tissue thyroid activity and is often influenced by concurrent NTI. It is, however, useful as part of a thyroid profile or health screening panel. For example, if levels of total T4, freeT4, and total T3 are all low, the patient more likely suffers from an NTI rather than hypothyroidism. If total T3 levels are high or very high in a dog not receiving thyroid supplementation, the patient most likely has a circulating T3 autoantibody (the most common type), which has spuriously [falsely] raised the T3 and/or freeT3 level.

Free T3 – As with freeT4, less than 0.1 percent of T3 molecules circulate freely in the blood and are biologically active. The blood’s freeT3 level tells the pituitary gland whether or not it needs to produce more TSH. Levels may be elevated slightly in euthyroid [normal thyroid function] dogs with increased tissue metabolic demands, and are typically spuriously high or very high in dogs with T3 autoantibodies. Both total T3 and freeT3 are typically normal in cases of hypothyroidism, unless the disease has been present and undiagnosed for some time, or the dog has concurrent NTI.

Thyroid Medication Administration

Dr. Dodds and colleagues also address the dosage of a companion dog’s thyroid medication on an individual basis. For a small consultation fee, additional concerns or questions can be submitted. However, remember that if your companion dog has not had the proper testing, you may need to go through the whole process again before determining the appropriate management and therapy. 

In the meantime, she suggests giving companion dogs levothyroxine (thyroid medication) in the following manner:

#1. Twice per day 

#2. 1 hour before OR 3 hours after feeding.

#3. Without foods or treats containing calcium or soy

#4. If calcium is not listed on the back panel of the treat, please refer to the manufacturer’s website

This illustrates an important distinction between canine and human hypothyroid treatment. Humans can take levothyroxine once per day – preferably first thing in the morning and withholding food for 2 hours.

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