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ENDOCRINOLOGY: HYPERTHYROIDISM IN CATS

YVCipedia ENDOCRINOLOGY
FELINE HYPERTHYROIDISM

Hyperthyroidism is production of an abnormally large amount of thyoid hormone by a tumor in the thyroid gland. It is a somewhat common problem of cats. Almost all affected cats are over 10 years of age; in our experience, most are 12 years or older at the time of diagnosis. 

The thyroid is a paired gland with one nodule on each side of the underside of neck. Hyperthyroidism is most often caused by a benign tumor in one of the two glands. Much less frequently the tumor is malignant and/or in both glands. 

Hyperthyroidism is a very slowly progressive problem. It usually takes at least many months and, more often, one or more years to produce significant signs.

SIGNS AND PROBLEMS ASSOCIATED WITH HYPERTHYROIDISM
Thyroid tumors produce an abnormally large amount of thyroid hormone. Thyroid hormone is normally involved, directly or indirectly, in most of the physiological processes of the body; that is, how the body functions. Having an excess amount of thyroid hormone results in all of these processes "speeding up". 

Cats with thyroid tumors usually have one or more of the following signs:
- increased appetite
- weight loss
- increased thirst and urination
- diarrhea
- restlessness, hyperactivity

Hypertrophic cardiomyopathy (HCM), a heart muscle problem, is another problem caused by hyperthyroidism. It in turn results in a rapid heart rate, and a predispostion toward the production of blood clots. The clotting problem is not common, but it can have devastating consequences for the pet.

Chronic kidney disease is another common problem of older cats. When a cat with chronic kidney disease develops hyperthyroidism, the hyperthyroid state improves blood flow to the kidneys, improving their function. When we successfully treat a cat for hyperthyroidism we might make a chronic kidney problem worse. This does not usually occur at a clinically significant level, but we are always careful to screen our patients for evidence of chronic kidney disease, and other problems, before treating them for hyperthyroidism.

DIAGNOSIS
We can often make our diagnosis with the physical exam. Thyroid tumors are often no larger than a large pea, but with our training and experience we can find them. 

Less commonly we make our diagnosis with blood testing. Still less commonly, some hyperthyroid cats do not have a tumor we can feel and have blood test results in a gray zone. These patients require additional testing to establish the diagnosis.

Complete general laboratory screening, with blood and urine profiles, xrays and sometimes ultrasound exam are needed to establish the over-all health status of the patient prior to treatment.


TREATMENT OPTIONS
DO NOTHING. For various reasons, some owners elect not treat medically or surgically. For some cats and their owners this is a reasonable and humane choice.

DAILY MEDICATION. Methimazole is a twice daily mediction that blocks the production of thyroid hormone from the tumor(s) in the gland. A small but significant percentage of cats have significant side effects, and  occasional blood testing is needed to monitor the patient's treatment. We do not consider this the best option, but it has worked well for some patients.

PRESCRIPTION DIET.  Prescription Diet Y/D is a complete and balanced diet that does not have iodine in it. Iodine is needed by the thyroid gland to make thyroid hormone; without it the gland cannot make the hormone. This treatment can work very well, but has one potentially major drawback: the pet has to eat the diet exclusively; even a little bit of other foods will provide iodine and render the Y/D ineffective.

SURGERY.  In our experience, for most cats, surgical removal of the thyroid tumor is the most practical treatment. This is a procedure that we have a lot of experience with, it is well tolerated by the pet; most cats go home the day of the surgery and do not need any special aftercare; and it almost always works. 

RADIOIODINE.  A dose of radioactive iodine is taken up selectively by the thyroid tumor and destroys it. This is perhaps the ideal treatment, except: A special facility is required for handling radioactive medications, and the cost is currently about $2000. Portland Vet Specialists is the only veterinary practice in Maine that is licensed to provide radioiodine treatment. Some additional information about this procedure appears below.

Yarmouth Veterinary Center
2014, updated 2015, 2016

FELINE HYPERTHYROIDISM: PORTLAND VET SPECIALISTS REQUIREMENTS FOR RADIOIODINE

Prior to going to Portland Vet Specialists for radioiodine treatment for hyperthyroidism, we need to complete the following tests at YVC:

    - CBC / Chemistry profile / Electrolytes blood test
    - Thyroid blood level by send-out to reference lab
    - Whole body lateral x-ray
    - Blood pressure measurement

These tests can be completed at one time or different times as long as they are all done within 2 months of the radioiodine therapy.

Patients must be off of fish-based foods for a minimum of 2 weeks prior to the blood testing. 

As of 2015, the cost of radioiodine therapy is about $1350. The laboratory testing, which is necessary, adds about $400.

Please see the article on hyperthyroidism and radioiodine therapy at www.portlandvetspecialists.com >>> services >>> radioiodine therapy >>>hyperthyroidism for more information. 

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