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Thyroid Hormone Testing in Dogs

Valid diagnostic testing is needed before thyroid diseases can be effectively managed. In this article, our Lincoln vets define thyroid testing, explain how it's done, and describe common types of tests. 

What is the thyroid gland?

Located near the trachea, the thyroid gland produces thyroxine (T4), a major thyroid hormone. These hormones affect many bodily functions by regulating metabolic rate. The pituitary gland is situated at the base of the brain and regulates the function of the thyroid gland with a hormone called TSH (thyroid stimulating hormone). 

What is thyroid testing?

A thyroid test is a type of blood test that assesses how the thyroid gland is working. Your veterinarian will likely recommend it for any sick cat or dog, and it's frequently used as a screening test for underlying disease or illness. If the results of this test fall within the normal range, this reveals information about your dog's health and excludes certain diseases as potential causes of your pooch's symptoms. 

If your dog tends to bleed excessively, the vet will take extra care after obtaining the sample to ensure no hemorrhaging from the site where the sample was obtained occurs. 

How is thyroid testing done in dogs?

During a thyroid test, a blood sample will be drawn from your dog and placed in a special glass tube, separating it into parts: serum and blood clot. The serum is is extracted and sent to a laboratory for testing, while the blood clot is discarded. While some animal hospitals perform thyroid tests in-house, most rely on outside laboratories. 

If it's performed at the animal hospital, a thyroid test typically takes between 40 and 60 minutes. If sent to an outside laboratory, you can expect the results to be returned within one or two days. 

Most dogs do not require anesthesia or sedation for the test. However, because some dogs dislike needles and react fearfully round them, anesthesia may be required in these circumstances. 

What are common types of thyroid tests?

Here are some of the most common thyroid tests done for dogs:

T4 & T3

Total T4 (Thyroxine) and Total T3 (Triiodothyronine) testing can be used to screen for hypothyroidism in dogs. Unexpectedly high levels of either hormone may be indicative of autoantibodies, and T3 and T4 concentrations can be influenced by a variety of factors including medications, disease states, and nutrition.

Free T4 by lmmulite or by Equilibrium Dialysis

A valid assay for measuring free T4 (FT4) can be used to distinguish true hypothyroidism from euthyroid sick condition. The non-protein bound thyroxine, FT4, is found in lower concentrations in the blood than total T4. A method should be used to separate the protein-bound hormone from the free (unbound) hormone for accurate FT4 testing.

The Equilibrium Dialysis (ED) method is the gold standard test for dogs, requiring an overnight incubation in buffer and dialysis cells to separate bound T4 from free T4. The Immulite method is less expensive and faster than the ED method, producing results comparable to dialysis.

Thyroid supplementation should be monitored using FT4 in any dog known or suspected to have thyroid autoantibodies, as these tests remove the autoantibody effects.

Thyroglobulin Autoantibody (TgAA) Test

The TgAA test is a canine-specific test for detecting autoimmune thyroiditis. For a more accurate diagnosis, it should be used in conjunction with other thyroid tests. Thyroglobulin autoantibodies are involved in the synthesis of T4 and T3.

TSH measurement

The endogenous thyroid stimulating hormone (TSH) can be measured in dogs. High levels of endogenous thyroid-stimulating hormone levels suggest hypothyroidism, but normal or low endogenous thyroid-stimulating hormone levels in dogs do not necessarily rule it out. This test should be used in conjunction with other thyroid tests to make a diagnosis.

Note: The advice provided in this post is intended for informational purposes and does not constitute medical advice regarding pets. For an accurate diagnosis of your pet's condition, please make an appointment with your vet.

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