Test Overview:

Congenital hypothyroidism is an inherited condition that is seen in several breeds of dog, and is generally inherited as an autosomal recessive trait. The causative mutation is different in each breed. Congenital hypothyroidism has also been known as cretinism.  In the toy fox terrier there is a mutation in the thyroid peroxidise gene, meaning that normal thyroid hormone is not able to be produced by the thyroid gland. In this breed the condition is called congenital hypothyroidism with goitre, because it is associated with gross enlargement of the thyroid gland.  Dogs with congenital hypothyroidism are affected from a day or two of age, and show reduced growth and less activity than their unaffected littermates. The head may appear to be enlarged. Within a few weeks, if they are still alive, they will show delayed eye and ear opening. The hair coat is abnormal and the limbs and face do not develop normally, giving the appearance of disproportionate dwarfism.  Most affected pups die or are euthanased by the age of 3 weeks. If given supplemental thyroid hormone from within the first few days of age pups can develop near normally, but in toy fox terriers the goitre will continue to develop, and will eventually cause respiratory obstruction. Mental retardation will be present unless thyroid supplementation is started within the first few days of life.  Many toy fox terrier breeders consider this a rare condition - although in one study the carrier rate was 31%. Breeders that may have had pups that have failed to grow and thrive and that have died in the first week or two of life may never have had them assessed or considered testing for congenital hypothyroidism with goitre. There is no cure for this condition, and although treatment is available and is partially successful, it is required daily for life, and needs to be started early in life to be useful. Almost all affected pups are euthanased within the first few weeks of life. Testing with the available DNA test and avoiding breeding two carriers together is the only way to avoid producing affected pups. Breeders should also be aiming to reduce the incidence of carriers in the population.