Normal thyroid levels for dogs depend on the age, breed and weight of the dog, states Vetinfo. Therefore, only a veterinarian can discern normal levels for specific dogs.
Thyroid levels are tested by measuring the thyroid hormones, explains Vetinfo. The thyroid hormones are T4, known as thyroxine, and T3, or triiodothyronine. A thyroid-stimulating hormone is also measured because it controls T4 and T3 hormone production. If TSH levels are abnormal, it is likely that the T4 and T3 levels are also abnormal. Abnormal thyroid levels indicate a possible autoimmune disease, a growth that is affecting the thyroid gland or even a tumor affecting the pituitary gland.