Thyroid Disease

The thyroid gland is a bi lobed structure in the front of the neck that is responsible for the manufacture and release of thyroid hormone.

The position is similar in dogs, cats, and people, but the more common diseases of this gland are different for different species.  For the sake of simplicity, I like to think of thyroid hormone as the regulator of metabolism. Too much thyroid hormone, and the metabolic rate goes up.  Too little, and the metabolic rate goes down.  

Today, let's concentrate on thyroid disease in cats.  We can talk about dogs next time.  Cats are prone to hyperthyroidism, an increase in circulating thyroid hormone.  It is a common disease in older cats caused by a benign tumor on the thyroid gland. If detected early, it can often be managed, or even cured.  Increased levels of thyroid hormone will cause an increased appetite, weight loss, rapid heart rate, and elevated blood pressure. Cats often present thin, often with a history of increased urination and intermittent, chronic vomiting. Most cases of this disease are easily detected with a blood test to measure the level of thyroid hormone in the blood, and if caught early, many of the complications of severe thyroid disease can be avoided.  One can imagine that prolonged, undiagnosed hormonal imbalances can cause serious disease. We see cats with heart and kidney disease secondary to uncontrolled hyperthyroidism.

Once diagnosed and underlying heath issues addressed, there are several options for treatment.  The gold standard is radioactive iodine therapy.  Performed at a specialty center, affected cats are given a single, painless injection of radioactive iodine.  This is preferentially taken up by the thyroid gland and it destroys the tissue.   The advantage is that cats that are good candidates for this therapy are essentially cured.  Ectopic thyroid tissue (tissue not found in the normal location) is also destroyed.  The down side is that cats need to be hospitalized in an isolation ward until their urine is no longer radioactive, usually 2 - 3 days, depending on your state's regulations.  It is also costly, and does require some pre treatment diagonsitics to make sure that this therapy is appropriate for your cat. Some cats become hypothyroid (low thyroid levels) after treatment. This condition is less serious but may require supplementation for the rest of the cat's life.

Radio iodine therapy has essentially replaced the less expensive, but riskier (and slightly less effective) surgical option of removing the thyroid glands.  This procedure, while still performed, requires general anesthesia.  In addition, if there is ectopic thyroid tissue, the surgery may not be effective.

Many clients opt for medical management. Methimazole, a relatively inexpensive medication, is given to the cat twice a day.  It blocks the incorporation of iodine into the active hormone at the sysnthesis level in the gland.  This effectively lowers the active hormone level and prevents the clinical disease from developing.  It does not remove the tumor, but since these are benign, that is not usually a problem.  The problem is that cats must be given medication twice a day for the rest of their lives and as we cat owners know, this may tax the owner/pet bond.  Most cats do not take kindly to taking medication.  Still, it is inexpensive and has the advantage of being reversible, that is if need be, the dose can be changed or the medication can be stopped.

This year, a new development in the treatment of hyperthyroidism in cats became available. Hills Pet Nutrition has launched a new iodine restricted diet for cats.  Since iodine is necessary to make thyroid hormone, this limited diet lowers the active hormone level in the blood.

 We are using this food for the three hyperthyroid cats in our hospital.  It has been well accepted in both the canned and dry forms and their thyroid levels seem to be normalizing, even off of their medication.  The food is not too limited, so other cats in the household can eat it, but we do recommend that they get a tablespoon of regular cat food on the side.  Cats that are being treated using this food can not eat any other food or treats because most of them contain iodine.

So make thyroid screening part of your cat's regular yearly health plan and if thyroid disease is found, early intervention will help your cat live a long and happy life.  You and your veterinarian can determine what type of therapy is best for your situation, but remember that early detection and intervention are definitely the best way to deal with hyperthyroidism.