Thyroid Gland in Dogs
The thyroid gland is located near the trachea and produces thyroxine (T4), a major thyroid hormone. Thyroid hormones greatly affect how the body functions by regulating the metabolic rate. Your dog's pituitary gland, located at the base of the brain, uses the TSH (thyroid stimulating hormone) to regulate the functioning of the thyroid gland.
Hyperthyroidism occurs when your dog's body produces an excessive amount of the thyroid hormone, increasing your dog's metabolic rate to dangerous levels. While this is rare in dogs, it's usually serious when it does appear, since it's primarily caused by a cancer called thyroid carcinoma.
Conversely, hypothyroidism occurs when your dog's body does not produce enough thyroid hormones, causing the metabolism to slow. Low thyroid levels impact all of a dog's organs. In most cases, the destruction of the thyroid gland causes hypothyroidism.
Thyroid Testing
A thyroid test is a blood test to see how well an animal's thyroid gland is working. Veterinarians often perform this test when a dog is ill to determine whether there is an underlying health condition. Normal results mean your pet is healthy and doesn't have certain thyroid-related diseases.
If your pet bleeds easily, it's important to take care to prevent more bleeding from the testing site where the sample was taken.
The Testing Process
To test your dog's thyroid, your vet will take a small blood sample, which is then collected into a special tube and separated into two parts, serum and a blood clot. While the serum is sent to a lab to be tested, the blood clot is thrown away. Some vet hospitals can perform this test onsite, which usually takes 40 to 60 minutes. If the sample is sent to an outside lab, you'll receive results in one or two days.
While most dogs won't require sedation, a few who are afraid of needles may need anesthesia.
Common Types of Thyroid Testing
Here are some common thyroid tests performed for dogs:
T4 & T3
Total T4 (thyroxine) and Total T3 (Triiodothyronine) testing can be done to screen for hypothyroidism, occurs in dogs with low thyroid hormone levels.
If levels of either hormone are unexpectedly high, this may point to autoantibodies. Several factors, including disease states, medications, and nutrition, can influence T3 and T4 concentrations.
Free T4 By Immulite or by Equilibrium Dialysis
A valid assay for measuring free T4 (FT4) can be used to distinguish true hypothyroidism from euthyroid 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
In dogs, thyroid function can be checked using a hormone called TSH. If a dog has high TSH levels, it could mean they have hypothyroidism. However, if a dog has normal or low TSH levels, it doesn't necessarily mean they don't have hypothyroidism. To be sure, it's best to use this test along with other thyroid tests when making 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.