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Hyperthyroidism: a condition in older cats

In our last article we described both how a normal thyroid gland works, as well as the condition of hypothyroidism (underactive thyroid). In veterinary medicine, hypothyroidism is a condition that almost exclusively affects dogs and hyperthyroidism (overactive thyroid) is a condition that almost exclusively affects cats. As a matter of fact, hyperthyroidism is currently the most common glandular disease diagnosed in cats.

These overactive thyroids are actually classified as a type of tumor called a functional adenoma. This can be misleading because when I was still in veterinary school my professors always taught me that anything that ended in "oma" was benign or noncancerous and anything that ended in "sarcoma" or "carcinoma" was malignant or cancerous.

A functional adenoma is a generally benign tumor which means it rarely metastasizes to other organs. However, it is functional which means it secretes some sort of hormone or chemical (in this case thyroid hormone). Remembering that thyroid hormone controls metabolism, an overproduction of thyroid hormone will speed metabolism. If untreated, this "hyped up" metabolism will lead to detrimental systemic effects and eventually death.

Hyperthyroidism affects older cats more commonly with an average age of diagnosis at approximately 13. There is not an increased risk depending on the gender of the cat, nor are certain breeds more at risk than others.

The signs of an overactive thyroid are related to an overactive metabolism. A majority of these cats will lose weight despite a voracious appetite, their fur will look rough and ragged, eyes sunken in, etc. Many of these patients will have an increased heart rate, occasionally a heart murmur and high blood pressure. A veterinary endocrinologist once told me to imagine drinking up to 20 espressos a day and that is what a hyperthyroid cat feels like all the time.

Testing for an overactive thyroid is relatively straightforward. Similar to testing for a hypo, or underactive thyroid in dogs, a T4 level (measurement of active thyroid hormone in the bloodstream) is a great screening test and is positive in 75 to 80 percent of hyperthyroid cats.

This is a little troubling because what do we do to confirm hyperthyroidism in the other 20 to 25 percent of cats that have all the symptoms of the disease but do not test positive initially? Fortunately, there is another test that improves our ability to pick up these tricky cases. A Free T4 by Equilibrium Dialysis (FT4ED) is a more sensitive way to pick up these trickier cases. There are other tests that are in development that should be available soon.

Treatment for hyperthyroidism consists usually of one of two options: methimazole (medication) or radioactive iodine treatment. A third option of surgically removing the affected thyroid gland is rarely used anymore. The cost of surgery is comparable with the cost of radioactive iodine treatment and there is a significant risk in anesthetizing a hyperthyroid cat for surgery.

Although a radioactive iodine treated cat must be isolated for three days post treatment to allow the cat to excrete the radioactive medication, no anesthesia is involved in this procedure. Approximately 10 percent of hyperthyroid cats treated with radioactive iodine become hypothyroid and required thyroid supplementation lifelong.

The medication methimazole is effective and relatively inexpensive, however it does require twice daily dosing lifelong, monitoring and adjustments in dosage periodically. The medication prevents the conversion of T3 (inactive thyroid hormone) to T4 (active thyroid hormone) but has no effect on the actual adenoma.

As these adenomas grow they will continue to produce more and more hormone and the dosage of methimazole needs to be adjusted to compensate. In my experience, the majority of cat owners choose methimazole treatment which is easier to budget for on a real time basis and would only choose radioactive iodine treatment for those patients that will just not take oral medication.

Hyperthyroidism in cats is a very treatable disease but other conditions can mimic a hyperthyroidism such as certain types of cancer or internal organ dysfunction. Therefore, please have your cat examined by your veterinarian before choosing an appropriate testing and treatment plan.

Dr. Kearns has been in practice for 12 years and is pictured with his son Matthew, as well as the newest member of the family, Jasmine, a Labrador retriever.

Matthew Kearns, DVM