Pituitary thyrotroph hyperplasia results from prolonged primary hypothyroidism in humans, mice and rats. In dogs with Cushing's disease, many cases have low serum thyroid hormones concentrations due to euthyroid sick syndrome. A 6-year-old castrated male Beagle diagnosed with Cushing's disease had a high serum thyroid stimulating hormone (TSH) concentration that was treated by hypophysectomy. On histological examination, the resected pituitary gland contained both a corticotroph adenoma and thyrotroph hyperplasia. The TSH-positive cell ratio in this case was greater than that of healthy Beagles. In the present case, the pituitary thyrotroph hyperplasia was probably caused by primary hypothyroidism. In conclusion, this Beagle is the first histological confirmation of the coexistence of a corticotroph adenoma and thyrotroph hyperplasia.