Hyperthyroidism in Cats

Hyperthyroidism is very rare in dogs but fairly common in cats.  Also called thyrotoxicosis, hyperthyroidism is caused by an increase in production of T3 and T4 (thyroid hormones) from an enlarged thyroid gland in a cat’s neck region.  In most cases, this is caused by an adenoma, a non-cancerous tumor.  Thyroid hormones affect nearly all of the organs in the body; therefore secondary problems often occur with hyperthyroidism. 

Symptoms of hyperthyroidism may be subtle at first but become more severe as the disease progresses.  The most common signs are weight loss and increased appetite, thirst, and urination.  You may also see vomiting, diarrhea, and a greasy, matted hair coat.   If hyperthyroidism is suspected, your veterinarian will conduct a physical exam of the cat’s neck region to check for an enlarged thyroid gland.  It is then likely he or she will order a blood chemistry panel and an analysis of thyroid hormone levels. 

There are four treatment options for feline hyperthyroidism, each having their own advantages and disadvantages and usually chosen based upon the patient’s overall health status, the owner’s ability to medicate the cat regularly, and financial considerations.  The four options include:  medication, radioactive iodine therapy, surgery, and dietary therapy.

Anti-thyroid medications are reasonably inexpensive and allow either short-term or long-term control of the disease.  However, life-long treatment of twice-daily medications will be required and some side-effects may be seen, including vomiting, anorexia, fever, anemia, and lethargy.

Radioactive iodine therapy is a highly-recommended treatment, as it is a procedure that often cures hyperthyroidism.  During treatment, radioactive iodine is administered as an injection and quickly absorbed into the bloodstream.  The iodine is taken up by the thyroid gland and the emitted radiation destroys abnormal thyroid tissue without damaging the surrounding tissue.  Most cats will have normal hormone levels within one to two weeks of treatment.  The disadvantage here is that this treatment is only permitted at facilities specially licensed to use radioisotopes; therefore not all veterinary clinics offer this treatment option.   

Surgery consists of the surgical removal of the thyroid glands, likely producing a permanent cure in most cats, eliminating the need for long-term medication.  The down-side is that it is more invasive and puts older cats at a higher risk as they undergo anesthesia.  Due to these reasons, it is a less likely choice for most owners. 

Dietary therapy is used to limit the amount of iodine in the diet.  Although prescription cat food is easily attained through most veterinary clinic, this tends to be more of a last resort for cats that are not candidates for other treatment options.  Long-term iodine restriction is concerning due to the effects on overall health and the possibility that such a diet may backfire and worsen the disease.  Research on this subject is presently ongoing. 

If left untreated, hyperthyroidism may cause heart disease and high blood pressure.  If you suspect thyroid disease in your cat, contact your veterinarian right away.  They will be happy to discuss your cat’s health and all treatment options with you to make sure the best route is chosen for YOUR pet.