This information is not meant to be a substitute for veterinary care. Always follow the instructions provided by your veterinarian. Hyperthyroidism is the overproduction of thyroid hormone by the thyroid glands.
Hyperthyroidism occurs most commonly in older cats and is rare in dogs.
There are 2 thyroid glands located in the neck. One or both of the glands can enlarge and overproduce thyroid hormone. Involvement of both glands is more common than involvement of one gland. Thyroid hormone affects the function of most organs in the body, so the signs of hyperthyroidism are quite variable. A diagnosis is made by feeling enlarged thyroid glands, increased blood levels of thyroid hormone or by a thyroid scan.