Feline Hyperthyroidism

            Feline hyperthyroidism is a disease characterized by a dramatically increased metabolic rate and the body’s inability to meet excessive metabolic demands.  It is most often due to overactive functioning tissue in one or both thyroid glands, although rare cases (1-2%) are due to thyroid cancer.  Hyperthyroidism is the most common endocrine  (hormone related) disease in cats and one of the most common diseases, in general, of middle aged to older cats.  The average age of onset is around 12 years.  It is equally common in males and females.

Typical clinical findings include:

1.             enlarged thyroid nodule

2.             increased appetite

3.             weight loss

4.             vomiting

5.             diarrhea

6.             hyperactivity

7.             increased thirst / urination

8.             heart murmur or arrhythmia

9.             elevated heart rate

10.         respiratory difficulty

These findings, while consistent with hyperthyroidism, can also be signs of similar disease processes or concurrent problems such as:

1.             heart disease

2.             chronic liver disease

3.             chronic kidney disease

4.             diabetes mellitus

5.             cancer

Because of these similarities, a diagnosis of feline hyperthyroidism needs to be made in light of other organ function levels.  This is why a CBC (complete blood count), a blood profile to check internal organ function, a urinalysis, and a thyroid test are often necessary to diagnose and define all aspects of this disease.

TREATMENT OPTIONS

            The options for the treatment of feline hyperthyroidism include choices between control and cure.  The disease can be controlled with lifelong daily medication.  Potential cure can be achieved with either surgery or radioactive iodine therapy.  These options are outlined below.  Regardless of the treatment method selected, periodic rechecks of the CBC, chemistry profile, urinalysis, and thyroid level may be required to assess the patient’s response to therapy. 

            Antithyroid drugs: Methimazole (Tapazole) can be administered on a daily basis to block the production of thyroid hormone, but is not curative.  The medication is available in pill form, as a flavored suspension, or as a gel that is applied to the skin.  All forms of the drug must be given one, two, or three times daily for life.  If administration of the medication is discontinued, clinical signs of the hyperthyroidism will return.  Some cats experience stomach upset with vomiting, diarrhea, and loss of appetite when on methimazole.  A less common side effect is severe facial itching.  These side effects may eliminate the medication as an option for treating the disease.  Cost for the medication is approximately $15 to $30 per month.

            Surgical Thyroidectomy: Surgical removal of the abnormal thyroid tissue often results in a permanent cure of the disease.  However, there is always the possibility that the remaining thyroid tissue may become overactive and the disease may return.  A second surgery to remove an abnormal gland on the opposite side of the neck may be considered since thyroid tissue elsewhere in the body is still active.  Risks of this surgical procedure include those associated with general anesthesia.  In a few cases, surgery may not be curative due to the presence of thyroid tissue in abnormal locations in the body.  The location of this abnormal tissue can only be determined with radioactive tracing studies. Cost for the thyroidectomy surgery is approximately $300 to $500.

            Radioactive Iodine Therapy: Another method to potentially cure hyperthyroidism involves the administration of radioactive iodine to destroy the abnormal thyroid tissue.  Lifelong cure is seen in 95 to 98% of cats after a single treatment.  Cats undergoing this treatment must be isolated at a special facility for a period of two days to three weeks.  The treatment is expensive (approximately $1000).  There is a very low incidence of side effects.

If you ever have questions regarding any of this information, please do not hesitate to contact us.  Visit us online at www.WhiteBearAnimalHospital.com.

WHITE BEAR ANIMAL HOSPITAL                           OTTER LAKE ANIMAL CARE CENTER

          1909 East County Road E                                                     6848 Otter Lake Road

       White Bear Lake, MN 55110                                                      Hugo, MN 55038

                  (651) 777-1393                                                                 (651) 426-8871