Thyroid carcinoma is a cancer of the thyroid gland, an endocrine organ consisting of two lobes situated on either side of the windpipe in the neck, which regulates metabolism. In dogs, unlike in cats, the vast majority (approximately 90%) of thyroid tumours are malignant. However, most canine thyroid carcinomas are non-functional, meaning they do not produce excess thyroid hormones and the dog remains euthyroid (normal hormone levels). A key prognostic distinction is whether the tumour is freely mobile under the skin or fixed to surrounding tissues: mobile tumours are typically well-encapsulated and can be completely removed surgically with an excellent prognosis, while fixed, invasive tumours have already grown into surrounding structures and are much more challenging to treat. Beagles, Golden Retrievers, and Siberian Huskies appear to be at somewhat increased risk.