Hyperthyroidism is a disease of older cats, caused by excessive production of thyroid hormones. The usual cause for increased hormone production is a tumor of the thyroid gland. These tumors may be benign (non-cancerous) or malignant (cancerous). The reason these glands develop tumors is not known.
Signs of excessive thyroid hormone production include weight loss despite a ravenous appetite, frequent bowel movements, increased thirst and urination, restlessness, frequent crying or vocalizing, neglect of normal grooming, and rapid heartbeat. The effects on the heart may be severe and result in congestive heart failure and death.
Treatment with radioactive iodine is an excellent form of therapy. This treatment, however, requires the availability of nuclear medicine facilities and isolation of the cat during treatment.
Drug therapy is also available. Some cats have problems with hyperthyroid medications, so monitoring with blood tests is necessary.
Surgical removal of the thyroid gland is sometimes effective for treatment. If the gland is totally removed, replacement thyroid hormone is given for life.