English Setters have the highest breed incidence of Autoimmune Thyroiditis, the inherited form of hypothyroidism that causes a dog’s immune system to attack the thyroid gland. A dog with Autoimmune Thyroiditis produces antibodies against the thyroid gland that can typically be detected in the blood starting sometime between 2 and 6 years of age.
Once a dog is producing these antibodies it usually takes years for the thyroid to be destroyed. Although some dogs will have symptoms of low thyroid at an early age most with the disease will only show signs later in life, and some never do. Some people believe the onset is triggered by infections, vaccines, or other environmental insults that irritate the immune system.
One study suggests hypothyroidism is caused by a single recessive gene, but this is not certain. 31.4% of the English Setters Michigan State University has tested through 2007 are positive for thyroid antibodies. The actual percentage of hypothyroid dogs may be even higher- only 54% of the English Setters submitted to the OFA’s thyroid database are normal.
The extremely high incidence combined with late onset of the disease make reducing hypothyroidism a daunting challenge in English Setters. With currently available testing it is impossible to eliminate all dogs that carry the genes. Many hypothyroid dogs will be well into their breeding careers before they test positive, and normal dogs that carry the gene may even be past breeding age by the time any of their offspring test positive. Until better testing methods become available progress in English Setters is likely to be very slow.
Although hypothyroidism is not as serious as some of the other inherited diseases, and can be treated effectively and inexpensively with medication, it can cause an incredible array of problems including ear, skin, and coat problems, lethargy, weight gain, reproductive failure, and seizures. It is a good idea to test the thyroid function of all English Setters periodically.
The best single screening test available for Autoimmune Thyroiditis is the thyroglobulin autoantibodies test (TgAA). This test is only reliable if it is started at a fairly early age and repeated regularly. Otherwise a full panel is necessary to evaluate the thyroid function. We test our breeding dogs for TgAA multiple times, beginning at two years of age.