All Pets Veterinary Clinic

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 amount 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 damaging nerves for the larynx. Also, there is an increased risk with 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 catís 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 from radioactive
treatment include cost and the necessity of isolating the animal at the treatment
facility until all of the radiation has cleared from the catís 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.



Karen Blakeley, DVM, MPH
14 December 2002