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Congenital Hypothyroidism (Discovered in the Toy Fox and Rat Terrier)

Congenital Hypothyroidism is a disease of insufficient thyroid hormone production. As this hormone is important in many aspects of the metabolism and development, the result is a wide variety of signs including slow growth, dwarfism, and mental impairment.

Found in

1 in 5,000 dogs

in our testing

Key Signs

Slow growth, Drowsiness, Disproportionate dwarfism, Goiter, Mental impairment, Abnormal hair coat

Age of Onset

0 to 2 yrs

Juvenile onset


Autosomal Recessive

For autosomal recessive disorders, dogs with two copies of the variant are at risk of developing the condition. Dogs with one copy of the variant are considered carriers and are usually not at risk of developing the disorder. However, carriers of some complex variants grouped in this category may be associated with a low risk of developing the disorder. Individuals with one or two copies may pass the disorder-associated variant to their puppies if bred.

Likelihood of the Condition

Moderate-high likelihood

At risk dogs are likely to show signs of this disease in their lifetime.

What to Do

Here’s how to care for a dog with Congenital Hypothyroidism

Partner with your veterinarian to make a plan regarding your dog’s well-being, including any insights provided through genetic testing. If your pet is at risk or is showing signs of this disorder, then the first step is to speak with your veterinarian.

For Veterinarians

Here’s what a vet needs to know about Congenital Hypothyroidism

Affected puppies appear normal at birth, with slowed growth usually observed around 3 to 8 weeks of age. Disproportionate dwarfism is typical for affected puppies: they have wide heads, short and thick necks, and short limbs. Affected puppies show mental impairment and their puppy coats are not replaced with the adult coat. Delayed opening of eyes and ear canals and delayed teething may be observed. Goiter (swelling and enlargement of the thyroid gland) is also observed. The disease may lead to death during the first weeks of life due to failure to thrive.

Most clinical signs will improve or be prevented if the condition is recognized early and thyroid replacement therapy is instituted. However, the thyroid glands may continue to expand despite treatment which could lead to airway obstruction.

For Breeders

Planning to breed a dog with this genetic variant?

There are many responsibilities to consider when breeding dogs. Regardless of test results it is important that your dog is in good general health and that you are in a position to care for the puppies if new responsible owners are not found. For first time or novice breeders, advice can be found at most kennel club websites.

This disease is autosomal recessive meaning that two copies of the mutation are needed for disease signs to be shown. A carrier dog with one copy of the Congenital Hypothyroidism mutation can be safely bred with a clear dog with no copies of the Congenital Hypothyroidism mutation. About half of the puppies will have one copy (carriers) and half will have no copies of the Congenital Hypothyroidism mutation. Puppies in a litter which is expected to contain carriers should be tested prior to breeding. Carrier to carrier matings are not advised as the resulting litter may contain affected puppies. Please note: It is possible that disease signs similar to the ones caused by the Congenital Hypothyroidism mutation could develop due to a different genetic or clinical cause.

Technical Details

Gene TPO
Variant C>T
Chromosome 17
Coordinate 784,660

All coordinates reference CanFam3.1

References & Credit

Credit to our scientific colleagues:

Bojanić, K., Acke, E., & Jones, B. R. (2011). Congenital hypothyroidism of dogs and cats: A review. New Zealand Veterinary Journal, 59(3), 115–122. View the article

Fyfe, J. C., Lynch, M., Olsen, J., & Louër, E. (2013). A thyroid peroxidase (TPO) mutation in dogs reveals a canid-specific gene structure. Mammalian Genome, 24(3–4), 127–133. View the article

Fyfe, J. C., Kampschmidt, K., Dang, V., Poteet, B. A., He, Q., Lowrie, C., … Fetro, V. M. (2003). Congenital Hypothyroidism with Goiter in Toy Fox Terriers. Journal of Veterinary Internal Medicine, 17(1), 50–57. View the article

Dodgson, S. E., Day, R., & Fyfe, J. C. (2012). Congenital Hypothyroidism with Goiter in Tenterfield Terriers. Journal of Veterinary Internal Medicine. View the article

Pettigrew, R., Fyfe, J. C., Gregory, B. L., Lipsitz, D., DeLahunta, A., Summers, B. A., & Shelton, G. D. (2007). CNS hypomyelination in rat terrier dogs with congenital goiter and a mutation in the thyroid peroxidase gene. Veterinary Pathology, 44(1), 50–56. View the article