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MaryDee Sist DVM
1629 Meech Road
Williamston, MI 48895


Interpreting Thyroid Test Results
by MaryDee Sist, DVM

Thyroid hormones are needed for the normal cellular metabolic functions of the body and therefore deficiencies affect almost all organ systems. Signs of a poorly functioning thyroid gland or HYPOTHYROIDISM have a gradual onset, are subtle and include metabolic (weight gain, mental dullness, heat seeking behavior), dermatological (poor coat, hair loss, seborrhea, pyoderma and myxedema or a puffy face and skin) and more rarely cardiovascular, neuromuscular (weakness) and reproductive (infertility) signs.

The major cause of hypothyroidism in dogs is autoimmune thyroiditis. It is an immune-mediated process that starts early in life and gradually progresses. In this condition, the dog produces Thyroglobulin autoantibodies (TgAA) that gradually destroy the functioning of the thyroid gland and the thyroid gland atrophies. It is only when the dog's thyroid gland is no longer capable of producing enough hormones to sustain clinical health that signs appear. The heritable nature of this disorder poses significant genetic implications for breeding stock.

There is no single easy test to diagnose hypothyroidism. Many veterinarians will run a T4 assay. If this value is within their established normal range, then most likely, the dog is not hypothyroid at this time. If the T4 is low, then further testing is definitely needed. Debilitating illnesses, certain drugs and even stress or advancing age can lower the T4 and the dog is not hypothyroid. In our Saluki study more than half of the dogs had T4 values lower than expected. It appears that the Saluki T4 average and range is normally lower than other breeds.

cTSH (canine thyroid stimulating hormone) is secreted in larger amounts by the pituitary gland when the T4 concentration falls. This stimulates the thyroid to make more T4 which prevent the signs of hypothyroidism from developing. If the TSH value is in the normal range, then in spite of the low T4, the dog is most likely not currently hypothyroid, as was the case for many of the Salukis sampled.

The new TgAA assay can identify dogs in the early compensatory stages of autoimmune thyroiditis long (even 3 to 5 years) before changes occur in the thyroid hormone levels or the signs of hypothyroidism become apparent. This affords important genetic and clinical options to be able to diagnose a heritable condition early on. The TgAA test alone is available from most commercial laboratories using serum and as a blood spot test from Oxford Laboratories.

OFA Thyroid Registry requires testing for these hormones. Continuing the certification of a normal status requires repeating these tests periodically as the dog ages. However, Free T4 (FT4D) is measured by dialysis on a specially handled sample by a certified laboratory because it is less influenced by physiological variations. The OFA Thyroid Registry certification categories are:

A. NORMAL MSU normal range
-FT4D within normal range 6-42 pmol/L
-cTSH within normal range 0-37 mU/L
-TgAA within normal range negative

-FT4D less than normal range <6
-cTSH greater than normal range >37
-TgAA positive positive

-FT4D within normal range 6-42
-cTSH greater than or within normal range 0-100
-TgAA positive positive

-FT4D less than normal range <6
-cTSH greater than normal range >37
-TgAA within normal range negative

-Any results that do not fit in the above categories

The OFA registry profile is designed for testing and certifying normal dogs for breeding purposes. For any dog showing clinical signs of hypothyroidism, having a full panel of thyroid values run is highly recommended. This panel should include both total and free measurements of T4 and T3, TSH as well as autoantibodies to T4, T3 and Thyroglobulin. These values should also be run to monitor and adjust thyroid supplementation on dogs that have been diagnosed and are being treated for hypothyroidism.

This full panel of thyroid values (TT4, TT3, TSH, FT4D, FT4, FT3, T4AA, T3AA and TgAA) was measured at the Endocrine Section of the Animal Health Diagnostic Laboratory, Michigan State University, on over 300 Saluki samples included in our study. The preliminary results showed that the Salukis had a lower normal range for T4 values when compared to the values generated from a mixed breed population and a low level of thyroglobulin autoantibodies. This suggests that autoimmune thyroidiitis and subsequent hypothyroidism is not a widespread problem in the Saluki breed. By breeding Salukis that are negative for TgAA we can continue this healthy trend in our unique breed.

The final statistical analyses are being processed and the results will be submitted for scientific publication.

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