Genetic basis - This condition in Shih Tzus is caused by a homozygous mutation in the SLC5A5 gene, which encodes the sodium/iodide symporter (NIS). The mutation is a single base transition (G > A) at the splice acceptor site in intron 9, resulting in improper splicing and defective iodide transport essential for thyroid hormone synthesis. It is inherited as an autosomal recessive trait, requiring two copies of the mutated gene to manifest the disease. Carriers (one copy) are asymptomatic but can pass the mutation to offspring.
Pathophysiology - Impaired iodide uptake by the thyroid gland leads to insufficient thyroid hormone production (hypothyroidism). The defective hormone synthesis causes goiter (enlarged thyroid gland) as the gland attempts to compensate by hypertrophy and hyperplasia. Clinical features can appear early, often within a few weeks after birth. Some affected dogs may also develop dilated cardiomyopathy, thyroid ectopic tissue, or follicular thyroid carcinoma. Unlike other hypothyroid forms, typical signs such as mental dullness, disproportionate dwarfism, and alopecia may be absent.
Complications - Swelling in the neck due to goiter. Mild heart murmurs and possible respiratory issues from goiter size. Retarded growth or mild growth delay. Exercise intolerance and coughing. Possible cardiac complications.
Why This Matters to Breeders and Vets - Breeders: Genetic testing is crucial to identify carriers and avoid breeding two carriers, preventing affected puppies. Vets: Early diagnosis can lead to supportive treatment with thyroid hormone replacement and management of associated complications. Awareness helps improve breed health and control this inherited disorder.