Hyperthyroidism is rare in dogs unlike cats, with thyroid carcinoma being the most common cause, requiring a different diagnostic and treatment approach than in feline medicine.
Canine thyroid carcinoma causes welfare harm through mass effects from the tumor itself, systemic effects of excess thyroid hormone, and the potential for metastatic spread. The metabolic effects of hyperthyroidism - increased heart rate, weight loss despite good appetite, and cardiovascular changes - cause ongoing physiological stress. Large thyroid carcinomas can cause pressure on the trachea creating breathing difficulty. The cancer nature of most canine cases means longer-term management and monitoring are required than for the typically benign feline condition. Early identification and appropriate treatment significantly improves welfare outcomes.