Hyperthyroidism in Cats
The thyroid is a gland that lies just below the larynx (voice box) with separated lobes on both sides of the trachea (windpipe).  Its function is to produce hormones that maintain the normal speed of function of organs throughout the body.  Thyroid hormone excess, hyperthyroidism, causes organs to overwork at excessive speed.

Causes and Incidence
Hyperthyroidism is the most common hormone disease in cats.  It is caused by a tumor of the thyroid gland.  More than 95% are benign.  The reason for the growth of tumors in the thyroid is not known, but the risk is higher in cats that eat canned cat foods.  The disease occurs in cats that are older than 8 years.  The average age is 13 years.

Signs
The most typical sign of hyperthyroidism is weight loss while maintaining a good appetite.  Other signs include excessive activity and nervousness, increased thirst and urination, and increased volume of feces.  Difficulty in breathing may result form the heart rate becoming rapid and the heart muscle becoming abnormally thick or weakened from excessive activity.  There is often a decreased desire to groom, and the hair coat becomes greasy-looking and unkempt.

Diagnosis
Your veterinarian can have your cat’s blood level of thyroid hormones measured.  If hyperthyroidism is discovered, he or she can evaluate your cat for the presence of complications or concurrent diseases.  The veterinarian will be able to aid your selection of the best method of treatment.

Treatment

There are three methods of treating hyperthyroidism.  The best treatment option for most cats is radioactive iodine to destroy the thyroid tumors.  This eliminates the need to perform surgery and is usually a permanent cure.  Radioactive iodine is administered as an injection without anesthesia.  After receiving radioactive iodine, cats must be confined to a radiotherapy isolation ward in a veterinary hospital for a few days.  After the radioactive iodine has been sufficiently eliminated from the cat’s body, the cat can be released from the hospital to go home.

A second form of treatment is to surgically remove the thyroid tumors.  Surgery can be curative, eliminating the disease.  However, cats with hyperthyroidism can have other health problems that can make them a poor surgical risk.  Also, other structures adjacent to the thyroid, such as the parathyroid glands, may become injured during the surgery.  Parathyroid hormone is essential for maintaining normal calcium levels in the blood.  A deficiency of parathyroid hormone can cause death.

The third choice for treatment is a drug that blocks the thyroid tumor from producing excessive hormone.  Unfortunately, drug effects only persist for 12 hours requiring an owner to give pills twice per day indefinitely.  Many cats do not tolerate daily pilling for the remainder of their life, and the pills can have adverse side effects in up to 20% of cases.  Drugs may be used for short periods to evaluate kidney function and to improve heart function prior to administering radioactive iodine or surgically removing thyroid tumors.

For more information, contact your local veterinarian or the MU Veterinary Medical Teaching Hospital 573-882-7821 or visit their web site.