Science Chemistry
Author: admin
Knowing More About the Science Behind Canine Thyroid Testing by Bryan Abram Marks
Effective canine thyroid testing is composed of the following tests, which all play a role in screening dogs for thyroid disorder:
Total T4. This test measures the total amount of T4 (thyroxine) hormone circulating in the blood-both bound and unbound molecules. More than 99% 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 (e.g. certain drugs). T4 is still the most popular and widely used initial screening test for thyroid disorder in dogs. As explained below in “The T4 Myth,” relying on the accuracy and sensitivity of this test alone is at the heart of the rampant 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. Another essential component of canine thyroid testing is serum free T4, which represents the tiny fraction (< 0.1%) of thyroxine hormone that is unbound and therefore is biologically active. As the free T4 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 Thyroid Stimulating Hormone (TSH). Although both the bound and free forms of T4 hormone are in circulation, the pituitary gland only recognizes the free molecule. Since protein and certain drug levels in the blood do not (or only minimally) affect free T4, it is considered a more accurate test of true thyroid activity than the total T4. Free T4 is much less likely to be influenced by NTI or drugs. Both total T4 and free T4 are lowered in cases of hypothyroidism. While endocrinologists may favor the equilibrium dialysis (ED) RIA method for measuring free T4 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 do not require radioisotopes and so, are environmentally “green” and 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. This form of canine thyroid testing is, however, useful as part of a thyroid profile or health screening panel. For example, if levels of total T4, free T4, and total T3 are all quite 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 free T3 level.
Free T3. As with free T4, less than 0.1% of T3 molecules circulate freely in the blood and are biologically active. The blood’s free T3 level tells the pituitary gland whether or not it needs to produce more TSH. Levels of both total and free T3 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 free T3 are typically normal in cases of hypothyroidism, unless the disease has been present and undiagnosed for some time, or the dog has concurrent NTI.
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. As discussed earlier, as much as 90% 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 their normalcy, or to obtain certification with health registries such as the OFA Thyroid Registry or Thyroid GOLDâ
Tags: science chemistry, science chemistry articles, science chemistry experiments, science chemistry games, science chemistry projects
