Hyperthyroidism is the excessive secretion of the thyroid hormones, T4 and T3, results in signs that reflect an increased metabolic rate and produces clinical hyperthyroidism. It is most common in middle-aged to old cats.

Hyperthyroidism is a common ailment in older cats. There are many clinical signs associated with feline hyperthyroidism. The hallmark sign is weight loss despite excellent appetite. Hyperthyroid cats may also exhibit gastrointestinal or cardiovascular signs. In the normal cat, the lobes of the thyroid gland cannot be felt with one’s fingers. In the hyperthyroid cat at least one lobe is usually prominent and may be detected during a physical exam.

There are several types of treatment for Feline Hyperthyroidism including radioiodine therapy, surgical thyroidectomy, or chronic administration of an anti-thyroid drug. Your veterinarian will discuss with you the best treatment options for your cat. 

VeterinaryPartner.com provides a general overview in this article, about Hyperthyroidism including its signs, symptoms and diagnosis

 Source: The Merck Veterinary Manual