The disclosure includes methods of identifying a dog at risk of developing a an autoimmune disease or condition, for example a hypothyroid disease or condition, comprising testing whether the dog exhibits one or more selected single nucleotide polymorphisms (SNPs), together with diagnostic kits for carrying out such methods, methods of treatment or prophylaxis of such autoimmune disease or condition, e.g., comprising administering an effective amount of tea extract to a dog in need thereof, and a canine diet or supplement comprising tea extract, useful for treatment of prophylaxis of such autoimmune disease or condition, or for maintenance of thyroid health in a dog.
