Hypothyroidism: Tests and Treatments

There are a number of tests used to measure how the thyroid gland is functioning. How these tests are interpreted and how their values interact gives the veterinarian and dog owner a clearer picture of how the thyroid gland is functioning. Currently, there are eight tests or parameters that are commomnly available for veterinarians to use when attempting to diagnose a thyroid condition. Each test looks at a different aspect of the thyroid system.

Total T4 (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 often the only test used. 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 TT4 result indicates hypothyroidism, but does not tell you why the dog is hypothyroid. Understanding why the dog is hypothyroid is necessary for finding the correct treatment.

Total T3 (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 enters the cell and is converted to T3 before the cell can use it.

Free T4 (FT4): In order to be effective, thyroid hormone must first enter the cell. To do this, it must not be bound to proteins circulating in the blood. Bound hormone is prevented from entering the cell, and therefore is not biologically available to the cells to use. TT4 measures both free and bound amounts of T4 in the blood. FT4 measures only the amount that is actually available to the cells. While a dog can have a TT4 that is in the normal range, if it is all bound so the body can't use it, the dog will still have the symptoms of hypothyroidism. The Cadillac of thyroid 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's cells.

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 hormone 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 hormones. A positive TgAA indicates the presence of hereditary autoimmune 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 thyroid hormone.

The most common form of hypothyroidism in dogs is lymphocytic thyroiditis, where the body's own immune system creates antibodies that attack and gradually destroy the thyroid gland. 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 TgAA result, will eventually develop low thyroid. Supplementing with T4 can sometimes slow this process down, although the mechanism for this is not completely understood. T3 or T4 autoiantibodies can cause false values of the free and bound T3 and T4 tests, unless FT4 is obtained by 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 pituatary gland is working overtime to stimulate the thyroid gland to produce hormone. Supplementing with T4, will again allow this biofeedback mechanism to return to normal.

The least common form of hypothyroidism is what is termed "sick euthyroid". Dogs with sick euthyroid will have a normal TSH, 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, and the other underlying (possibly serious) medical condition will go undiagnosed and untreated.

Stress, certain medications, and some other diseases (like Cushing's ) can cause thyroid levels to drop temporarily. Correctly identifying the source of hypothyroidism will help to select appropriate treatment for the dog. In some cases, a dog may need supplementation for only a short time until the thyroid production process normalizes. Unlike with other hormonal systems, supplementation with thyroid hormone does not shut 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 veterinarian determine whether a dog needs more, less, or no supplementation? This is done by splitting the T4 dosage and monitoring serum concentrations of T4. For instance, if a dog needs 0.8 mg of thyroid per day, it should get 0.4 mg in the morning, and 0.4 mg in the evening. 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 adjustments need to be made to the dog's supplementation level. The follow-up panel should include TSH, TT4 and FT4D, at a minimum. In dogs that are on long-term thyroid medication, serum values should be tested every six months just to be sure they are still receiving adequate supplementation.

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 of 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 of T4 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 the normal range seems to keep symnptoms from appearing. Many individuals who have low normal levels do not seem to do as well as if they are supplemented up into the upper normal level.

OFA now certifies dogs for thyroid function. Since OFA's main concern is genetic thyroid problems, 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 from their regular vet. Your vet can draw the blood, and send the tubes, Application for Thyroid Database form, and certification check to the OFA-Approved laboratory of your choice. While the panel allows dogs to become certified if the results are normal, it is not a diagnostic panel. If a dog is showing symptoms of hypothyroidism, then a more detailed thyroid panel test should be done, as described above.

The interpretation of OFA's panel is as follows:

• Normal = FT4D & TSH normal, TgAA negative.
• Positive Autoimmune Thyroiditis = FT4D below normal, TSH above normal, TgAA positive
• Positive Compensative Thyroiditis = FT4D normal, TSH above normal, TgAA positive
• Idiopathically reduced thyroid function = FT4D below normal, TSH greater than or equal to normal ranges, TgAA negative

The thyroid panel submitted to OFA will come back with one of these four diagnoses. A fifth possible outcome is Equivocal, with the recommendation that the tests be repeated in three to six months. In addition, OFA recommends recertification of normal dogs every year for the first four years, as TgAA usually appears by four years of age. After four years of age, the recommendation is to repeat the panel every two years. Dogs must be at least one year old in order to be certified.

(c) 2002 Lisa Van Loo, revised (c) 2008

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