Congenital hypothyroidism with goitre due to dyshormonogenesis and an iodine organification defect, caused by a nonsense mutation in the TPO gene. Affected puppies are born with goitre, inactivity, abnormal coat and delayed development; it is lethal without early diagnosis and treatment. It is inherited in an autosomal recessive manner and the same variant is present in the Rat Terrier and Toy Fox Terrier. A complementary test, not a substitute, for clinical examination.

Documented in the Rat Terrier and Toy Fox Terrier (and in the Tenterfield Terrier with another variant). Selection programmes based on carrier testing reduce its incidence; no current published frequencies.

- Bilateral goitre palpable from the first weeks of life
- Inactivity, failure to suckle and growth retardation
- Abnormal coat, stenotic ear canals and delayed eye opening
- Low total and free T4 and very high TSH
- Central nervous system hypomyelination in the Rat Terrier
- Recovery of growth with early hormone replacement therapy

In 2003, Fyfe and colleagues described in the Toy Fox Terrier a congenital hypothyroidism with goitre that segregated as a simple autosomal recessive trait, with an iodine organification defect and a nonsense mutation in the TPO gene; a carrier test was developed. In 2007, Pettigrew and colleagues described the same condition in 3 of 5 Rat Terrier puppies, with central nervous system hypomyelination and the same TPO mutation, which had presumably passed to the Rat Terrier from the Toy Fox Terrier through crosses intended to reduce size.

- Test breeding animals with the TPO c.331C>T test before mating
- Do not mate two carriers: 25 % affected homozygotes
- A carrier may be mated to a clear animal; test the offspring intended for breeding
- Exclude affected animals and known carriers from breeding
- Guarantee early hormone treatment in any affected puppy (the condition is lethal without it)

The differential diagnosis includes other causes of congenital hypothyroidism (dyshormonogenesis due to other genes, thyroid dysgenesis, acquired causes). In the Rat Terrier it may be accompanied by central nervous system hypomyelination. Treatment with levothyroxine almost completely restores development. Do not use other breeds' test for this variant.

1. Fyfe JC et al. 2003, Congenital hypothyroidism with goiter in toy fox terriers. J Vet Intern Med 17(1):50-7. PMID: 12564727. 2. Pettigrew R et al. 2007, CNS hypomyelination in Rat Terrier dogs with congenital goiter and a mutation in the thyroid peroxidase gene. Vet Pathol 44(1):50-6. PMID: 17197623. 3. Fyfe JC et al. 2013, A thyroid peroxidase (TPO) mutation in dogs reveals a canid-specific gene structure. Mamm Genome 24(3-4):127-33. PMID: 23223904. OMIA:000536-9615.