Total T4 (or TT4): TT4 measures the total circulating level of T4 thyroid hormone in the blood. This is the most common test for thyroid function, and is, unfortunately, often the only test used by vets when they check thyroid function. While it is useful as part of an overall profile, it is impossible to diagnose or rule out hypothyroidism by using the TT4 value alone. A low result on the TT4 indicates hypothyroidism, but does not tell WHY the dog is hypothyroid. The "why" is important for proper diagnosis and treating the underlying cause for the low value.
Total T3 (or TT3): This tests for circulating concentrations of T3 hormone. T3 hormone is the hormone that is actually used by the cells for most cellular regulation. T4 hormone enters the cell and is converted to T3 before being used.
Free T4 (FT4): In order to function, thyroid hormone must first enter the cells. To do this, it must not be "bound" to proteins. There are proteins circulating in the blood that thyroid hormones will attach themselves to. These bound hormones cannot enter the cell to do their job. A total T4 will measure both free and bound amounts added together. The free T4 test will measure ONLY the amount of T4 hormone that is actually biologically available for the cells to use. While a dog can have a normal Total T4, if it is all bound so the body can't use it, the dog still will have the symptoms of hypothyroidism. The "Cadillac" of T4 tests is the free T4 by equilibrium dialysis method (FT4D). This test gives a very accurate picture of the amount of T4 available for use by the body.
Free T3 (FT3): This tests for circulating concentrations of unbound T3, or the biologically available T3 hormone.
Thyroid Stimulating Hormone (TSH): This test measures the amount of TSH in the blood. TSH is secreted by the pituitary gland, and stimulates the thyroid gland to produce more thyroid hormone. The TSH value relative to other thyroid values indicates possible causes for hypothyroidism.
Thyroglobulin Auto-Antibody (TgAA): This test tells whether the body's immune system has developed antibodies against the thyroid gland. There are also specific antibody tests for antibodies against T3 and T4. A positive TGAA indicates the presence of hereditary thyroid disease.
The results of these tests, when evaluated relative to each other, give a good picture of the function of the entire thyroid system, pituitary gland, thyroid gland, and how the body uses the thyroid hormone.
The most common form of hypothyroidism in dogs is lymphocytic thyroiditis, where the body's immune system creates antibodies that attack the thyroid gland, slowly destroying it. This process occurs over several years, so a dog with a normal T4 value as a young dog will eventually develop hypothyroidism, as it grows older. A dog with normal values on all the tests, but a positive value for Thyroglobulin auto-antibodies (TgAA), will eventually develop low thyroid. Supplementing with T4 can sometimes slow down this process, although the mechanism for this is not completely understood. T3 auto-antibodies or T4 auto-antibodies can cause false values of the free and bound T3 & T4 values, unless free T4 was obtained using the dialysis method, in which case the FT4D will be the true value. The presence of any of these antibodies indicates lymphocytic thyroiditis, which is believed to be hereditary in nature.
In dogs that have primary hypothyroidism but not lymphocytic thyroiditis, the T4 value will be low or borderline low (low normal) with a high TSH level, and normal TgAA. This indicates that the pituitary gland is working "overtime" to stimulate the thyroid gland to produce thyroid hormone. Supplementing with T4, again, will allow this biofeedback mechanism to return to normal.
The least common form of hypothyroidism is what is termed "sick euthyroid". These dogs will have a normal TSH value, a normal FT4D, and normal TgAA, yet still show symptoms of hypothyroidism. These dogs are developing signs of hypothyroidism as a result of some other disease process in the body that may be interfering with thyroid hormone protein binding, metabolism, or something else. It is important to run the FT4D to correctly diagnose these animals, as the regular T4 test may show such animals as hypothyroid when they actually are not. Identifying the underlying medical condition and treating that will correct the thyroid symptoms as well. Unfortunately, if the correct diagnostic thyroid test is not performed, then the dog could be misdiagnosed as hypothyroid, when in fact it is not, and the other underlying (possibly serious) medical condition will go undiagnosed and untreated.
Stress, certain medications, and some other diseases (such as Cushing's) can cause thyroid levels to drop temporarily. Correctly identifying the source of the hypothyroidism will help to select appropriate treatment for the dog. In some cases, a dog may need supplementation for a short time until the thyroid production process normalizes. Unlike with other hormonal systems, supplementation with thyroid hormones does not shut down natural production capabilities of the thyroid gland. Once a dog with a temporary thyroid condition returns to normal, that dog will, once again, be able to produce adequate amounts of thyroid hormone, without supplementation.
How do the owner and vet determine whether a dog needs more, less or no supplementation? This is done by splitting the T4 dosage and monitoring serum concentrations. For instance, if a dog needs .8 mg of thyroid a day, it should get .4 in the AM, .4 in the PM. This allows a fairly steady amount of thyroid hormone to be present throughout the day. Approximately six weeks after supplementation is started, another panel should be run, to see what changes have occurred, and if any adjustment needs to be made in the dog's supplement levels. The follow-up panel should include TSH, TT4 and FT4D. In dogs that are on long-term thyroid medication, their serum values should be tested every six months just to be sure they are still receiving adequate therapy.
Values for these tests depend on how soon the blood is drawn after the medication is given. For instance, if you have your dog's blood drawn for the test within three hours after having its thyroid pill, the T4 value will be in the high normal range. If the blood is drawn just before the next pill is due, the value may be in the low normal range. Owners should strive to keep blood levels within the upper half of the normal range. Possibly because of the increased haircoat demands of our breed, maintaining levels in the upper half of normal seems to keep symptoms from appearing. Many individuals who have low normal levels do not seem to do as well as if they are supplemented into the upper normal range.
OFA now certifies dogs for thyroid function. Since OFA's main concern is genetic thyroiditis, they focus on the results of three tests, FT4D, TSH and TgAA. If a breeder is interested in getting their otherwise healthy dogs certified, he or she can request the OFA thyroid panel, and have them run by one of the OFA-approved laboratories (see OFA's website, http://www.offa.org/thycert.html for a list of approved laboratories). While this panel will allow a dog to become certified if it is normal, it is not a good diagnostic panel if a dog is showing symptoms of hypothyroidism and a diagnosis needs to be made. That would require the more extensive tests listed above.
The interpretation of OFA's panel is as follows: Normal=FT4D and TSH both within normal range, TgAA negative. Positive autoimmune thyroiditis=FT4D below normal, TSH above normal and positive TgAA. Positive compensative autoimmune thyroiditis=FT4D normal, TSH greater than normal and TgAA positive. Idiopathically reduced thyroid function=FT4D below normal, TSH greater than or equal to normal range, TgAA negative. The thyroid panel submitted to OFA will come back with one of these four diagnoses. A fifth result would be listed as "equivocal", with the recommendation that the tests be repeated in three to six months. In addition, OFA recommends re-certification of normal dogs every year for the first four years, as TgAA usually appears within the first four years. After four years of age, the recommendation is to repeat the panel every two years. Dogs must be at least one year of age in order to be certified.
As for Gwenny, the rescue dog, her supplementation began immediately, and within a month, she had become a different dog. Her weight distribution changed, as her fat disappeared and she developed more muscle. Where she had been unable to move because of the fluid in and around her lungs and heart, she began to lose that fluid and became more willing to go for walks. Her attitude improved, and she was more interested in things going on around her. She also was less quarrelsome with the other dogs. Overall, the thyroid supplementation resulted in a rejuvenation, and her last six months was spent in much better physical comfort, as well as mental enjoyment.
‹2002 Lisa Van Loo