Autoimmune or lymphocytic thyroiditis is the main pathologic process causing hypothyroidism as an adult-onset condition in dogs. Affected dogs have normal thyroid function at birth and grow in a normal manner. Then a response is triggered where the body's immune system reacts to the thyroid follicles (structures in the thyroid glands that produce the thyroid hormones) as foreign tissue. The immune system attacks and ultimately destroys the follicles with the end stage of an irreversible loss of functional thyroid tissue. Thyroglobulin is a large protein made by the cells in the thyroid follicle that contains thyroid hormone and serve as the storage reservoir. The thyroglobulin autoantibodies (TgAA) measured in the laboratory are a product of the immune response in the thyroid gland. From the start of this pathologic process, it will be months to years, until the end stage of hypothyroidism is reached. There is a growing body of data indicating that some euthyroid sight hound breeds can have lower T4 values than laboratory reference ranges established from a variety of dog breeds.

The objective of this survey was to assess prevalence of different categories of thyroid function or pathology based on combinations of total thyroxine (T4), thyroid stimulating hormone (TSH) and thyroglobulin autoantibody (TgAA) results in Rhodesian ridgeback, borzoi, greyhound and saluki dogs. Submissions to the Diagnostic Center for Population and Animal Health (College of Veterinary Medicine, Michigan State University) for thyroid testing from 1/1/2005 through 1/1/2009 were searched. Results were tabulated with respect to laboratory reference ranges derived from multiple pure bred and mixed breed normal results.

Table 1. Percentage of samples by breed of dog for each category of total thyroxine (T4), thyroid stimulating hormone (TSH), and thyroglobulin autoantibody (TgAA) results

Normal T4 15-67 nmol/L, low T4 <15 nmol/L
Normal TSH < 37 mU/L, elevated TSH > 37 mU/L
Negative TgAA < 25%, elevated TgAA > 25%

The finding of T4, TSH and TgAA within the laboratory reference range supports a diagnosis of euthyroidism or normal functioning of the thyroid gland. 67.1% Rhodesian ridgebacks, 53.2% borzoi, 34.8% greyhounds and 43.6% salukis were within the general laboratory reference ranges for T4, TSH and thyroglobulin autoantibodies. The finding of T4 values below the normal range with a normal TSH and negative TgAA was highest in the greyhound 57.7%, saluki 32.1%, borzoi 30.3% and only 9.1% in Rhodesian ridgebacks. However, it is well recognized in greyhounds and salukis that T4 values in euthyroid dogs extend below general laboratory references ranges. If normal TSH, negative TgAA and normal or low T4 values were combined, 92.5% of greyhounds, 75.7% of the salukis and 83.5% of the borzoi would be considered euthyroid.

Low T4 and elevated TSH with either negative or positive thyroglobulin autoantibodies provide optimal support for a diagnosis of hypothyroidism, if consistent with the clinical signs. 5.7% Rhodesian ridgebacks, 4.4% borzoi, 10.7% salukis would be classified as hypothyroid, but this was found in only 2.9% of the greyhound samples.

Positive thyroglobulin autoantibodies indicate autoimmune or lymphocytic thyroiditis, which is inherited. Positive TgAA results were found in a total of 16.8% of Rhodesian ridgebacks, 11.5% of borzoi, 12.8% of saluki and 3.1% of the greyhound samples. In samples with a low T4 and elevated TSH, Rhodesian ridgebacks were more likely to have positive thyroglobulin autoantibodies. The overall low prevalence of thyroglobulin autoantibodies suggests that hypothyroidism and/or autoimmune thyroiditis is uncommon in greyhounds.

The incidence of autoimmune thyroiditis and the subsequent development of clinical hypothyroidism vary significantly among sight hound breeds. Early testing for thyroglobulin autoantibodies (TgAA) provides valuable information for breeders wishing to decrease the incidence of clinical hypothyroidism through selective breeding. Breed specific reference ranges should be considered when interpreting thyroid test results and diagnosing hypothyroidism in sight hounds.