The thyroid gland controls the metabolic rate of the body. When the gland functions insufficiently, a condition known as hypothyroidism occurs. Dogs that are clinically affected may display one or more of the following clinical signs: weakness, lethargy, weight gain to the point of obesity, skin and coat problems, behavioral abnormalities, and infertility. Breed health surveys tell us that hypothyroidism is one of the most common health concerns expressed by breeders.

Canine hypothyroidism is frequently misunderstood, misdiagnosed, and mistreated. Historically, it has been thought that 50 percent of cases of canine hypothyroidism are caused by autoimmune thyroiditis, and the rest are caused by idiopathic hypothyroidism. What the experts now understand is that almost all primary hypothyroidism in dogs is caused by thyroiditis (autoimmune destruction of the thyroid gland), and that this is a genetic disorder. Primary idiopathic hypothyroidism, if it exists at all, is a rare condition. The confusion comes from looking at blood test "snapshots" of hypothyroid dogs, and not understanding the whole "moving picture" of thyroid disease.

Measurable antibodies to the thyroid gland and to thyroid hormones develop in the blood of dogs affected by autoimmune thyroiditis. For months to years, these hormones attack and gradually destroy the normal thyroid gland tissue. It is only after a large portion of the thyroid gland is destroyed that the levels of thyroid hormone in the bloodstream drop. It is at this time that the clinical signs of hypothyroidism mentioned above may appear. Once the thyroid gland is destroyed, the body is no longer stimulated to produce the antithyroid antibodies. The dog is now in end-stage hypothyroidism. Most erroneous diagnoses of primary idiopathic hypothyroidism occur because the blood test is performed at this stage, when the gland is already destroyed and the autoantibodies are gone. Thus, the process that has led to this point is not seen.

In a study by Dr. Raymond Nachreiner and his colleagues Michigan State University, more than 50,000 canine blood samples have been screened for significant levels of autoantibodies to either thyroglobulin (TgAA), thyroid hormone 3 (T3AA), or thyroid hormone 4 (T4AA). Of the blood samples tested, 7.9% tested positive for thyroid autoantibodies. Dogs younger than two years of age tested positive in less than 5 percent of the samples, while the tests were positive between 9 and 11.5 percent of the time for between 2 and 6 years of age. The highest percentage of positive tests occurred at 4 years of age. The peak age for low thyroid hormone levels and no autoantibodies was eight years.

There are metabolic, infectious, endocrinologic, and cancerous illnesses that have no autoimmune components but which can nonetheless cause low thyroid-hormone values. This problem, which occurs less frequently is generally referred to as secondary hypothyroidism. It is not a hereditary thyroid disorder.

There is some controversy as to whether environmental toxins or vaccines cause autoimmune thyroiditis. These act as stresses on a dog's body and could possibly affect the onset or severity of autoimmune thyroiditis. However, Dr. W. Jean Dodds, founder of the animal blood bank Hemopet, states that only dogs that have the genetic potential can develop autoimmune thyroiditis. Therefore, any dog that has significant levels of blood-thyroid autoantibodies is considered genetically affected with hypothyroidism, and to carry a gene (or genes) that cause the disorder. Dr. Dodds also reports that thyroid supplementation may be protective to dogs with thyroid autoantibodies even before their thyroid hormone levels drop.

We now know that measuring autoantibodies is the best available way to diagnose hereditary hypothyroidism. The Orthopedic Foundation for Animals thyroid-registry database states: "As a result of the variable onset of the presence of autoantibodies, periodic testing will be necessary.... Since the majority of affected dogs will have autoantibodies by 4 years of age, annual testing for the first 4 years is recommended. After that, testing every other year should suffice. Any test showing significant levels of thyroid autoantibodies confirms a diagnosis of hereditary hypothyroidism."

Compounding the problem of just who has hereditary hypothyroidism are the myriad of conditions that can respond to thyroid supplementation. The clinical signs of hypothyroidism can appear in dogs that have conditions which are not related to thyroid problems, and these conditions may respond to thyroid supplementation. Just because a dog has a condition that responds to thyroid supplementation, it should not be assumed the dog has hypothyroidism. Unless autoantibodies or low thyroid hormone levels are found, most of these dogs are probably thyroid-normal.

Studies on the mode of inheritance of hereditary hypothyroidism/autoimmune thyroiditis in dogs have been inconclusive to date. What has been established is that some breeds have a much greater likelihood of developing autoimmune thyroiditis than do others, while some breeds have a below-average risk. (see tables). These breeds respectively carry a higher or lower genetic load of hypothyroidism causing genes. As opposed to human autoimmune thyroiditis (Hashimoto's disease), where there is a female to male 10-to -1 ratio, hypothyroidism affects male and female dogs about equally.

Research into hypothyroidism has concentrated on perfecting the diagnostic tests and, in affected dogs, outlining the progression of the disease. There is no active research at this time into the mode of inheritance of canine hereditary hypothyroidism. It is hoped that with more reliable data on which dogs are affected (producing autoantibodies at a young age) and the further development of canine genome screening, we can learn how to better control the disease through selective breeding. At this point, the recommendation that can be offered is the standard one for dealing with polygenic hereditary diseases: Breed normal-testing dogs that come from litters which have mostly tested normal.

Breeds with the highest prevalence (>9% affected) of hypothyroidism (Data from the endocrinology lab at Michigan State University)

English Setter, Dalmatian, Basenji, Rhodesian Ridgeback, Old English Sheepdog, Boxer, Maltese Dog, Chesapeake Bay Retriever, Beagle, Cocker Spaniel, Shetland Sheepdog, Siberian Husky, Border Collie, Husky, Akita, Golden Retriever.

Breeds with the lowest prevalence (<3% affected) of hypothyroidism (Data from the endocrinology lab at Michigan State University)

Chihuahua, Lhasa Apso, Pomeranian, Miniature Pinscher, Cairn Terrier, Basset Hound, Schnauzer, Yorkshire Terrier, Boston Terrier, Norwegian Elkhound, Greyhound, Portuguese Water Dog, Newfoundland, Bichon Frise, Welsh Corgi, Miniature Schnauzer, Cavalier King Charles Spaniel, Flat Coated Retriever.

(This article originally appeared in the "Healthy Dog" column of the AKC Gazette, August 2001.)