Chantal, after treatment

A friend’s cat was recently diagnosed as hyperthyroid; and my friend, an avid web surfer, found out about radioactive iodine (“I131“) as a treatment option. Her cat is 15 years old and in good body condition (meaning that she isn’t thin, as many hyperthyroid cats get). She wants to know if it’s a good therapeutic choice for her cat.  

My answer: A firm “maybe.”   

Hyperthyroidism in cats usually results from either benign tumors or hyperplasia (expansion) of the thyroid gland, which then produces too much thyroid hormone. This increase in circulating hormone can then lead to increased heart rate and high blood pressure, GI problems (appetite changes, diarrhea, vomiting), hyperactivity, heart disease (cardiomypathy), and kidney disease. These cats often lose weight and begin urinating excessively, which are the things that owners notice first. Left untreated, this is a progressive disease whose secondary effects usually become life threatening.   

Currently, the most widely accepted treatments for hyperthyroid cats are oral or transdermal medications (typically methimazole), surgical removal the thyroid gland, thyroid ablation (chemical destruction), or I131. The latter acts by being drawn into overly-active thyroid cells, which rapidly absorb iodine, and destroying them.   

The best thing about I131 is that it is a permanent solution to a debilitating disease; but its permanence is also its biggest threat. Why? Because hyperthyroidism in cats can mask other conditions, most typically kidney disease. In these cats, the increased blood flow through the kidneys (resulting from elevated thyroid function) makes the kidneys appear to function adequately, so that often only the thyroid condition is evident on blood work. Once blood flow returns to normal during thyroid treatment, impaired kidney function will become evident. If a kidney problem becomes evident, it is possible to use a small amount of medication to reduce the effects of hyperthyroidism while still allowing increased blood flow to the kidneys. If, however, a permanent solution is used first, there will be no way to protect the kidneys from the effects of the permanently reduced blood flow.   

So, in all cats diagnosed with hyperthyroidism, the first treatment should be oral or transdermal medications. Those cats that respond well to a medication should remain on it for a few months, until the thyroid levels are reduced to nearly normal and “masked” diseases can be identified. At that time, a decision to use one of the permanent therapies can be made. In that minority of cats that cannot tolerate thyroid medications, that decision will come earlier – and with less complete information. This is what happened to my own cat, Chantal, whose intolerance of methimazole caused us to use I131 without sufficient knowledge of masked disease. Unfortunately, she did have mild kidney insufficiency, which has progressed slowly since her 2003 treatment. But, since she’s now 22 years of age, I’d say she’s done pretty well.   

My advice to my friend: use medications long enough to rule out other diseases. After that, I131 is a great, though expensive, choice for your kitty.