The diagnosis and management of thyroid diseases in dogs need valid testing for it to be accurate. Here, our Lexington vets discuss thyroid hormone testing for dogs including how the testing is done, and the common types of tests.
Thyroid Gland in Dogs
The thyroid gland, which is located near the trachea, produces thyroxine (T4), a major thyroid hormone. Thyroid hormones have far-reaching effects on the body by regulating metabolic rate. The pituitary gland, located at the base of the brain, regulates thyroid gland function with a hormone called TSH (thyroid stimulating hormone).
A thyroid test is a blood test that evaluates the thyroid gland's function. It is recommended in any sick animal and is frequently used as a screening test for underlying illness or disease. Normal results aid in determining health and excluding certain diseases.
If the animal tends to have excessive bleeding, extra care should be taken after obtaining the sample to ensure no hemorrhaging from the site where the sample was obtained.
The Testing Process
A thyroid test requires drawing a blood sample, placing it in a special glass tube, and separating it into two parts: serum and blood clot. The serum is extracted and sent to a laboratory for testing, while the blood clot is discarded. Some veterinary hospitals can perform thyroid tests in-house, but most rely on outside laboratories.
If performed at the veterinary hospital, a thyroid test usually takes about 40–60 minutes. If given to an outside laboratory, you can expect the results within 1–2 days.
Most dogs do not require sedation or anesthesia. Some dogs, however, dislike needles and may require anesthesia.
Common Types of Thyroid Testing
The following are some common thyroid tests performed 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 the 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.
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.