Kale is not acutely toxic to dogs, but it contains several naturally occurring compounds — oxalates, isothiocyanates, and calcium oxalate — that can cause digestive upset and, with large or repeated servings, more serious issues including kidney irritation and thyroid interference. Small amounts of plain kale fed occasionally are unlikely to harm a healthy adult dog. Owners of dogs with kidney disease, hypothyroidism, or a history of bladder stones should avoid kale entirely.
Clinical note: Goitrogenic & oxalate-containing vegetable — safe in strict moderation for healthy dogs