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Feline Hyperthyroidism

By Dr. Rochelle Campbell

Hyperthyroidism is one of the most common endocrine diseases found in cats.  It is extremely rare in dogs (dogs usually tend to develop hypothyroidism).  This condition usually occurs as a result of a nodule that grows on the thyroid gland itself.  This nodule secretes thyroid hormones and this hormone release is unregulated by normal physiologic influences.  It usually occurs in late middle-aged and older cats.  Cats with hyperthyroidism typically present for a physical exam because their owners notice several clinical signs, including weight loss (despite a ravenous appetite), hyperactivity, behavioral changes and sometimes vomiting and diarrhea.   A veterinary exam on a hyperthyroid cat may reveal a large thyroid gland, thin body condition, an increased heart rate, possible heart murmur, thick nails and an unkempt appearance.  If your veterinarian suspects hyperthyroidism, he or she may recommend one or more blood tests to help confirm a diagnosis.  If bloodwork results reveal that the thyroid hormone level is high, then your veterinarian will discuss treatment options for your cat.

There are several options available for treatment.  I131 treatment (Radioactive iodine treatment) uses radioactive iodine to destroy the abnormal tissue on the thyroid gland and, will eliminate the need for daily, long term treatment with oral medication.  Your veterinarian will refer you to a facility that specifically performs this type of therapy as special precautions and handling need to be taken when performing this particular treatment.  Daily oral medication, such as Methimazole, that works to lower thyroid hormone levels (by specifically blocking thyroid hormone synthesis) is another effective option.  These types of medications can also occasionally be compounded into a topical paste that can be absorbed through the skin should it be too difficult to medicate a cat orally.  The oral route, however, should be the first choice if feasible.  A newer option for treatment is Science Diet y/d.  Studies have shown that this diet, if fed exclusively to a hyperthyroid cat, can lower thyroid hormone levels.  Surgical removal of the thyroid gland was once considered a preferred method of treatment but now the other treatment options mentioned above are preferred over surgery.  All of the treatments will involve follow up appointments as well as monitoring bloodwork periodically.  If you suspect your cat has hyperthyroidism, it is best to get your pet to the veterinarian as soon as possible so treatment will not be delayed.