The incidence of hypothyroidism in cats after radioiodine (RAI) therapy and how to best treat hypothyroidism is being actively studied by a number of researchers. This means that recommendations are changing as our knowledge expands.

The rate of hypothyroidism does not seem to be related to thyroid hormone level but may be related to the dose RAI given. Institutions that use a fixed dose seem to have a higher incidence of hypothyroidism than facilities that use a scaled dose. We have been using a scaled dose of RAI since 2001 which may account for our low rate of hypothyroidism.

There may be a slightly higher incidence of hypothyroidism for cats which are on antithyroid drug therapy and are well controlled at the time RAI is given. If cats have low or normal thyroid hormone levels with drug therapy, normal tissue may be active. If cats are given RAI at this time, normal tissue may take up the RAI. This may contribute to hypothyroidism and is why we try to have cats off antithyroid medications at least 7 days prior to RAI therapy. We want the cats to be hyperthyroid so that normal tissue is inactive at the time of treatment and does not take up the RAI.

In some cats hypothyroidism is likely related to how much of the gland has become hyperthyroid. If 99% of the gland has been converted to tumour, then the remaining 1% of normal cells will likely not be able to produce enough hormone to reach adequate levels.

Hypothyroidism, in contrast to hyperthyroidism, is not a life threatening disease. However, hypothyroidism (whether on drug therapy or post-RAI) can cause kidney disease. Signs of hypothyroidism include lethargy, weight gain and poor hair coat. Hypothyroidism in cats does not correlate well with thyroid hormone levels. Since other diseases can suppress (or elevate in the case of fT4 levels) thyroid hormone levels, we will request a TSH level to make sure your cat is actually hypothyroid before starting supplementation. We do not want to treat a cat that does not need treatment! In cats without kidney disease we prefer to wait at least 3 months after RAI to start supplementation as some cats require this long to reactivate normal thyroid tissue.

Treating hypothyroidism is easier to treat than hyperthyroidism. Since we are dealing with a non-life-threatening condition, the need for medication is less urgent. Missing a few doses of thyroxine means your cat may start feeling sleepier. Missing just one dose of methimazole can result in racing heart rates and damage to the body.

Thyroxine (hormone replacement) is best given twice daily although some cats do well on once daily therapy. In cats with kidney disease we recommend twice daily treatment to best support their kidney function. Thyroxine is odourless and tasteless so it can be mixed with canned food or a treat instead of giving your cat a pill. Methimazole is extremely bitter and many cats will stop eating it in food within a few weeks.

As many humans are hypothyroid, thyroxine is a relatively inexpensive medication. Methimazole often needs to be formulated into liquids, chew tabs or gels in order to medicate your cat which increases its cost.

Once the correct dose of thyroxine is found for your cat, that is his/her dose for life. Rarely, after a year or more, cats may regain normal thyroid function and can be weaned off supplemental hormone. In contrast, when on methimazole, thyroid hormone levels should be checked at least every 3 months to ensure the dose is still correct. Cats require methimazole for the rest of their lives (unless cured with RAI).

There are no side effects to thyroxine whereas the numerous side effects of methimazole are listed in the “other treatment options” section.

Although we do not wish any cat to become hypothyroid post RAI therapy, the reality is it happens in a small percentage of cats. However, just as in human medicine where the incidence of hypothyroidism is as high as 50%, specialists agree that eliminating a hard to control, life threatening disease (hyperthyroidism) and creating an easily treatable, non life threatening condition (hypothyroidism) is the healthiest approach. Even if the incidence of hypothyroidism were 100%, RAI would still be recommended as the treatment of choice.