Surgery is no longer commonly performed for routine treatment of hyperthyroidism but may be considered as adjunctive therapy in very large tumours or adenocarcinomas.

Surgery cures the hyperthyroid condition by removing part or all of the thyroid gland. This requires anesthesia, and postoperative complications are not uncommon.

There are increased risks during anesthesia for hyperthyroid cats. Surgery itself also carries risks, such as potential damage to the laryngeal nerves and parathyroid glands.

Laryngeal nerve damage can result in life-threatening breathing problems. Some cats with laryngeal nerve damage have required tracheotomies post surgery in an attempt to save their lives.

The parathyroid glands are important in calcium metabolism, and up to 34% of cats will have some problems associated with low blood calcium after surgery. Symptoms range from weakness or tremors to seizures which may require days of hospitalization, monitoring, and calcium supplementation – potentially for life.

Up to 80% of cats have abnormal tissue in both glands at the time of diagnosis, even though only one gland may be enlarged. In these cats, hyperthyroidism will recur within 18 months after surgery. Thyroid tissue can be present in other places in the body as well, in which case hyperthyroidism will persist or recur even after surgery.

Cats that undergo surgery where both thyroid glands are removed have almost a 100% chance of requiring thyroxine supplementation. Only approximately 10% of cats require thyroxine supplementation after radioiodine therapy.