Congenital hypothyroidism (CHG) in Rat Terriers and Toy Fox Terriers

Reduced thyroid function (hypothyroidism) is one of the most common endocrine diseases affecting both humans and many animals, including dogs. It is a complex clinical condition that is caused by a combination of genetic and environmental factors.

In primary congenital hypothyroidism, the onset of clinical symptoms is early (within one week of age). Symptoms include lethargy, inability to suck, growth retardation (dwarfism), vertebral and limb dysplasia, thickened subcutaneous tissue, delayed teething, delayed opening of the eyes and ear canals, and delayed hair growth. Thyroid cells undergo hypertrophy and hyperplasia, resulting in goitre (enlarged thyroid gland).

The cause of CHG in rat terriers and toy fox terriers is a nonsense mutation c.331C>T in the gene for thyroid peroxidase (TPO). TPO is the enzyme responsible for binding iodide to thyroglobulin, a necessary step in thyroid hormone synthesis.

The mutation is inherited in an autosomal recessive manner. This means that only individuals who inherit the mutated gene from both parents will develop the disease. Carriers of the mutated gene are clinically healthy but pass the mutation on to their offspring. In the case of a mating between two heterozygous individuals, theoretically 25% of the offspring will be completely healthy, 50% of the offspring will be carriers and 25% of the offspring will inherit the mutated gene from both parents and will therefore be affected by the disease.

The genetic test can clearly reveal the genotype of the animal and is a useful tool for breeders to avoid unintentional breeding of affected puppies.

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References:

Fyfe, J.C., Kampschmidt, K., Dang, V., Poteet, B.A., He, Q., Lowrie, C., Graham, P.A., Fetro, V.M. : Congenital hypothyroidism with goiter in toy fox terriers Journal of Veterinary Internal Medicine 17:50-7, 2003. Pubmed reference: 12564727