Hypothyroidism, insufficient production of hormone by the thyroid glands, is the most common endocrine disorder affecting dogs, It's common in certain breeds, but not all cases of canine hypothyroidism are genetic. However, a 2004 study funded by the American Kennel Club Foundation for Canine Health generated data on hypothyroidism incidence in numerous breeds, noting a "clear genetic component."
The thyroid glands lie on either side of the trachea. These small glands produce hormones regulating metabolism. When not enough hormone circulates in the body, all sorts of problems arise. Hypothyroidism falls into three categories. The most common, primary hypothyroidism, only concern the thyroid glands. Secondary hypothyroidism involves the pituitary gland and tertiary hypothyroidism involves the hypothalamus. A fourth, congenital, type causes cretinism in puppies, most of which are euthanized because of severe deformities. This fourth type is known to occur in the fox terrier.
The breeds most commonly diagnosed with hypothyroidism include the Airedale, Doberman pinscher, fox terrier, golden retriever, Labrador retriever, cocker spaniel, dachshund, miniature schnauzer, English setter, Irish setter, Rottweiler, mastiff, German shepherd, Newfoundland, Rhodesian Ridgeback and Scottish deerhound. Hypothyroidism more often affects larger and medium-size dogs rather than in small breeds. Symptoms begin appearing between the ages of 2 and 5, although onset varies by breed. It's not unusual for signs to initially show up in middle-age or older canines. Although the disease affects both genders equally, spayed females tend to develop hypothyroidism at higher rates than intact females, according to the Merck Veterinary Manual.
Because the thyroid glands regulate a dog's metabolism, hypothyroidism affects canines in various ways. Common symptoms include hair loss, weight gain, lethargy, skin discoloration, cold and exercise intolerance and skin thickening. By the time symptoms become apparent, much of the thyroid gland is no longer functioning. Treatment generally consists of administering synthetic thyroid hormone to the affected dog for the rest of his life.
Autoantibodies are proteins produced by the dog against one of his body's own proteins. According to the Indiana-based Advanced Animal Imaging, development of thyroid autoantibodies at any point in a dog’s life indicates that the animal likely has the genetic form of the disease. AAI recommends testing susceptible breeds every year for autoantibodies until the age of 4, since the status can change. Once the dog is past that age, every-other-year testing is sufficient.