Hyperthyroidism

The thyroid gland is a paired gland located in the neck area of cats and dogs. It is responsible for the secretion of a hormone called thyroxine. When the thyroid gland excretes excessive amounts of thyroxine, a condition called hyperthyroidism occurs. Currently, hyperthyroidism is the number one endocrine disease seen in cats. The remainder of this article will discuss the occurrence, signs, diagnosis, and treatment of hyperthyroidism.



Occurrence. Hyperthyroidism is generally a disease of middle-aged or older cats. The average age is 12, but cats have been diagnosed as hyperthyroid as young as 5-6 years and as old as 20 years. Hyperthyroidism occurs equally in male and female cats.

Signs. Cats with hyperthyroidism can show many signs. Some of them include weight loss despite an increased appetite, increased thirst, increased urination, restlessness or hyperactivity, an increased heart rate, heart murmur, vomiting, diarrhea, panting, muscle tremors, and poor hair coat. In addition, in most hyperthyroid cats, an enlarged thyroid gland can be felt.

Diagnosis. A simple blood test to determine the number of thyroid hormones being secreted can confirm if a cat is suffering from hyperthyroidism. However, the blood tests will not determine if one or both of the thyroid glands is/are causing the problem. A test called a nuclear scan can be done to determine if the cat has a problem in one or both of the thyroid glands. Knowing which gland(s) is/are affected becomes important if surgery is going to be the method of treatment chosen. In addition to thyroid hormone level determination, a complete blood count, chemistry panel, urinalysis, and cardiac workup may also be done.

Treatment. Currently, there are three methods of treatment for hyperthyroidism in cats. Each has distinct advantages and disadvantages. First of all, is medical management. An antithyroid drug called methimazole may be given to a hyperthyroid cat. This drug will block the production of thyroid hormone and thus signs of disease will be eliminated. This method of treatment is advantageous because it is easy and somewhat inexpensive. However, medical management does not cure the disease, it simply keeps it under control. Thus, the medication often needs to be given for the lifetime of the 

cat. There is also the possibility of some side effects from the medication.

The second method of treatment is surgical. If the increased secretion of thyroid hormone is due to a thyroid secreting tumor, the thyroid gland(s) can be removed. The main disadvantages of surgical treatment include the risk of leaving behind some abnormal thyroid tissue and thus having the disease reoccur, the risk of damaging the parathyroid glands, glands that are found very close to the thyroid glands, and the risk of damaging nerves for the larynx. Also, there is an increased risk of anesthesia since hyperthyroid cats are generally older cats.

The third method of treatment is radioactive iodine therapy. With this method, radioactive iodine is injected into the cat's veins. The iodine will selectively concentrate in the thyroid gland. Thus the radiation will destroy the hyperactive thyroid tissue. This method of treatment is currently considered the best option for most hyperthyroid cats. The main advantages include no need to give daily medication for the life of the cat, no risks of surgery, no chance of life-threatening surgical complications, and no risk from anesthesia. The major disadvantages of radioactive treatment include cost and the necessity of isolating the animal at the treatment facility until all of the radiation has cleared from the cat's body (generally 1-2 weeks).

The information provided is for educational purposes only and is not intended to take the place of your regular veterinarian. Please do not hesitate to contact your regular veterinarian if you have questions regarding your pet.