Dalmatians Are Ranked Among Breeds Susceptible to Hypothyroidism
Hypothyroidism, the end stage of lymphocytic thyroiditis, or autoimmune thyroid disease, is the most common endocrine disorder in dogs. Considered a hereditary condition, hypothyroidism affects virtually all dog breeds and even mixed breeds.
Dalmatians, Beagles, German Wirehaired Pointers, and Maltese rank sixth among all breeds for hypothyroidism, based on samples received for thyroid testing at MichiganStateUniversity’s DiagnosticCenter for Population and Animal Health.¹ The ranking reflects the percentage of dogs of a particular breed that tested antibody positive for hypothyroidism. English Setters had the highest percentage of dogs that tested positive.
Meanwhile, based on thyroid test results submitted to the Canine Thyroid Registry at the Orthopedic Foundation for Animals through December 2012, Dalmatians also ranked sixth among all breeds in which results were provided.
Kent Refsal, D.V.M., Ph.D., professor of endocrinology at the DiagnosticCenter for Population and Animal Health at MichiganStateUniversity, says, “The disease progression in most Dalmatians starts with a normal thyroid test at a young age. Elevated autoantibodies are the first abnormality that occurs, though affected dogs otherwise are healthy.
“As destruction of the thyroid follicles progresses, the capacity of the thyroid gland to make T4 and T3 hormones diminishes, prompting an increase of TSH (thyroid stimulating hormone) from the pituitary gland in an attempt to compensate. With almost complete destruction of the thyroid follicles, thyroid hormones finally become low in the blood circulation. When destruction of the follicles is complete, an immune response declines and autoantibody results become normal or negative. This process may span several years.”
Owners may elect to have either a thyroid or eye disease test for Dalmatians to receive certification through the Canine Health Information Center (CHIC). Required tests are hips and BAER hearing. Breeders and owners must use a laboratory approved by the Orthopedic Foundation for Animals for thyroid screening. Thyroid testing should be performed at 2, 3, 4, 6 and 8 years of age for continued certification.
Subsidized health clinics held at the DCA National Specialty for several years have included thyroid testing, Garvin says. “We also encourage owners to pursue CHIC certification,” he says. “To further promote health and well-being, DCA gives up to $1,100 in bonus prize money to Futurity winners whose sire and dam both have CHIC certification and a discount on ads in Spotter magazine to people whose dogs are CHIC certified.”
Testing for Thyroid Disease
The thyroid, located on the throat below the larynx, produces the hormones thyroxine (T4) and triiodothyonine (T3), which control metabolism and the oxygen consumption of most every tissue in the body. Dogs with hypothyroidism have metabolic rates below normal, which usually is caused by lymphocytic thyroiditis. Idiopathic thyroid gland atrophy is a rare cause of hypothyroidism.
Typically, dogs are born with normal thyroid function. The autoimmune response is triggered, attacking and destroying the cells of the thyroid follicles, the portion of the gland that makes thyroid hormones. An assessment of thyroid function is based on three variables:
- The total or free thyroid hormones, T4 and T3, in the circulation, which gives insights on hormone production from the thyroid gland
- The thyroid stimulating hormone (TSH), indicating the pituitary gland is signaling for an increase in production of thyroid hormones from the thyroid gland
- Thyroid autoantibodies (TgAA), a marker for lymphocytic thyroiditis
“The anti-thyroglobulin autoantibody (TgAA) test remains the most sensitive screening for lymphocytic thyroiditis,” says Duncan Ferguson, V.M.D., Ph.D., DACVIM, DACVCP, head of Veterinary Biosciences at the University of Illinois College of Veterinary Medicine. “It has been shown to be positive one to two years before abnormalities in free T4 or TSH. By the time T4 concentration falls, the thyroid gland may be about 75 percent destroyed by autoimmune thyroiditis.”
“Many endocrine specialists, including me, believe that perhaps hypothyroidism is overdiagnosed following poor understanding of the diagnostic tests. Hypothyroidism is tough to properly diagnose when it is a differential diagnosis for a variety of clinical signs including skin problems, reproductive issues and overt neurological problems. Thyroid hormone concentrations can be perturbed by other nonthyroidal conditions as well. For the most part, TSH does not rise and remain elevated over repeat sampling in many situations other than true hypothyroidism.”
Refsal agrees that testing is not a straightforward process. “Experimental studies in healthy dogs show that giving glucocorticoids or phenobarbital can decrease thyroid hormones. In these cases, decreases of thyroid hormones are usually not accompanied by an increase of TSH. Treatment with sulfonamides will have a direct effect on the thyroid gland to interrupt thyroid hormone production, whereas the decrease of thyroid hormones is accompanied by an increase of TSH. However, thyroid function will return to normal after sulfonamide treatment is discontinued. Further, results from clinical and experimental studies show that TSH results are not elevated in all hypothyroid dogs.”
Dogs suffering from some non-thyroid illnesses may have low thyroid hormones when tested, as their illnesses can affect normal levels of T3 and T4. Euthyroid sick syndrome results from chronic, debilitating conditions such as diabetes, congestive heart failure, hepatic disease or even acute conditions like renal failure. When the underlying disorder is treated, the thyroid condition returns to normal.
Treatment for hypothyroidism involves giving a veterinarian-prescribed synthetic version of the T4 hormone, called thyroxine, and having blood tests taken periodically throughout a dog’s life to monitor the supplemented levels. Untreated dogs rarely die from hypothyroidism, but they are at greater risk for infection and obesity, which can strain the heart.
Breeding Against Hypothyroidism
Most hypothyroidism occurs in adult dogs between 4 and 6 years of age. A breeder may have already bred a dog when he discovers the hypothyroidism. Dogs that test positive for hypothyroidism should be removed from breeding programs.
“The limited studies from test matings and pedigree analyses have shown that lymphocytic thyroiditis is a heritable trait in dogs that possibly has an autosomal recessive mode of inheritance,” Refsal says. “It can be frustrating because abnormalities in thyroid test results may not appear until dogs are older.
“If affected dogs are eliminated from breeding programs, their parents and at least some siblings remain as carriers,” he continues. “If widespread selection programs are implemented to rid a breed of lymphocytic thyroiditis, there is a chance that some desirable traits will be reduced and other heritable conditions that are more severe and debilitating could emerge. If a breeder has an affected dog, I think that alternative replacements should be sought, if at all possible. The choice comes down to perpetuating lymphocytic thyroiditis or risk bringing in another less-desirable heritable condition.”
Although hypothyroidism is not curable, dogs can be successfully treated and few die from the disorder. Increased awareness will help breeders and owners recognize signs of the disease.
Signs of Hypothyroidism
Hypothyroidism can be difficult to detect in dogs, partly because dogs don’t always show signs of the disorder. Here are common signs.
- Excessive weight gain with no corresponding increase in appetite
- Lethargy, including sleeping more than usual, lack of interest in play and tiring on regular walks
- Hair loss, particularly behind the ears and on the tail and flanks
- Dry, flaky skin
- Greasy, seborrheic skin
- Intolerance of cold temperatures shown by excessive shivering and constant seeking of warm places to lie down
- Slowed heart rate
- Difficulty swallowing
- Muscle weakness, stiffness and lameness
- Chronic ear or skin infections
- Chronic gastrointestinal disorders, including constipation
¹Graham PA, Refsal KR, Nachreiner FR. Etiopathologic Findings of Canine Hypothyroidism. Veterinary Clinics of North American Small Animal Practice. 2001;37(4):617-631.