C130 - Congenital Hypothyroidism with Goiter

Congenital Hypothyroidism with Goiter (CHG) in Toy Fox Terriers is caused by lack of thyroid peroxidase (TPO), an enzyme in the thyroid gland responsible for adding iodine to a protein called thyroglobulin. A mutation in the TPO gene prevents production of the enzyme and the consequent inability to produce thyroid hormones. Thyroid hormone is essential for normal development and metabolism in dogs, especially during the rapid growth period that puppies experience. Pupies affected with CHG are abnormal from just a few days of age. They do not move around as much as normal puppies, and the head may appear large in comparison to the body. By the second week of age, a swelling on the underside of the neck can be felt and continues to enlarge with time. The delay in lengthening of bones in the legs, spine, and face causes dwarfism. Mostly all abnormalities are alleviated by early diagnosis and daily oral administration of thyroid hormone medication, but this does not stop the goiter from continuing to enlarge and constricting the airway.

Recent studies have demonstrated that Congenital Hypothyoidism with Goiter is inherited as a simple autosomal recessive trait in Toy Fox Terriers. Therefore, a puppy affected with Congenital Hypothyridism with Goiter must have received a mutated copy of the disease gene from both parents. Male and female puppies are equally affected. The parents of affected pups show no outward signs of disease, but they are obligate carriers by definition. This means that in a breeding program, both male and female carriers will pass on their mutant alleles to 50% of all their offspring.

Because there is such a broad range of possible clinical signs, hypothyroidism can be quite difficult to diagnose solely on clinical findings. Blood tests may show certain suggestive abnormalities, but are often non-specific. Fortunately, identification of the TPO mutation has allowed the design of a new laboratory test to detect the mutation associated with CHG in DNA from blood or cheek cells from Toy Fox Terriers. DNA analysis of the TPO gene determines whether a dog is affected with the disease or is a carrier of the mutation. Unidentified carriers in breeding programs continue to spread the mutant allele throughout the Toy Fox Terriers breed. Using this test, breeders can determine the genetic status of their animals and use this information to eradicate this disease from their lines.