For today’s blog post, I wanted to discuss a common disease we diagnose and manage in older cats. Feline hyperthyroidism is defined as increased levels of thyroid hormone circulating in the blood, typically produced by a growth on the thyroid gland. The thyroid gland is found in the neck, and is important for regulating multiple processes in the body, including the metabolism and heat production. It also helps with normal function of the kidneys, heart, respiratory, and gastrointestinal system. We do not fully understand why, but in many older cats a benign nodule grows on the thyroid gland that produces unregulated, excess amounts of thyroid hormone. A very small percentage of these growths can be cancerous, but the vast majority are benign. Depending on the source, it is estimated that 2-4% of all cats will develop hyperthyroidism during their lifetime.
The clinical signs we see with hyperthyroidism are related to the normal functions of the thyroid gland. Essentially, the thyroid gland does its normal job too well, resulting in a higher than typical metabolic rate and changes to the heart, kidneys, and gastrointestinal tract. Cats with hyperthyroidism tend to lose weight, despite an increased appetite. They are often more vocal, hyperactive, and do not groom as well. People often notice increased thirst and urination as well. When we examine cats with hyperthyroidism, they have commonly lost weight and muscle mass. They also tend to have changes when we listen to their heart, such as a faster heart rate, heart murmur, or arrhythmia. Alternatively, about 10-20% of cats with hyperthyroidism will act sick, with a poor appetite and lethargy.
If these changes have been noted at home in an older cat, we recommend a blood test to evaluate their thyroid hormone levels. An elevated thyroid hormone level (called thyroxine) is diagnostic for hyperthyroidism. Some cats in early stages of hyperthyroidism may have levels at the high end of the normal range, and we may need to pursue further testing to confirm the diagnosis.
For the vast majority of cats, hyperthyroidism is a treatable condition. The gold standard of therapy is called radioactive iodine treatment. Iodine is essential for the production of thyroid hormone. When radioactive iodine is given as an injection, this material is taken up into the thyroid gland and kills the tissue producing excessive levels of hormone. While therapy with radioactive material may sound scary, this treatment is generally very safe and well-tolerated. Cats do need to have this treatment performed in a specialized facility, and typically remain hospitalized for 1-2 weeks after treatment, until they are no longer radioactive. Radioactive iodine treatment is curative, meaning for about 95% of cats there will be no further treatment needed for their hyperthyroidism. For about 5% of cases, this treatment will work too well, resulting in hypothyroidism or a low thyroid hormone level. These cats will then need thyroid hormone supplementation. We know that cats treated with radioactive iodine have the best long-term prognosis, and it is our recommended treatment option when possible.
If we cannot pursue radioactive iodine therapy, our next treatment option is a medication called methimazole. This medication disrupts the production of thyroid hormone, working to bring it back to a normal level. When starting this medication, we need to monitor follow-up bloodwork to make sure the dose is appropriate to achieve the desired level of thyroid hormone. Methimazole is well-tolerated, with rare instances of side effects including gastrointestinal upset, facial itching, liver changes, and low blood cell counts. Methimazole is typically given as a twice-daily oral medication, but can be compounded into a transdermal medication for cats who are tricky to medicate. Methimazole therapy does require lifelong medication and regular bloodwork monitoring to keep hyperthyroidism controlled.
Our final option for treating hyperthyroidism is an iodine-restricted diet. As we discussed, iodine is essential for the production of thyroid hormone, so if we can restrict iodine intake we can lower the level of hormone produced. There is currently only one diet produced with low iodine levels – Hill’s y/d canned or kibble food. Dietary therapy is generally our last choice as it tends to not be as effective at controlling hyperthyroidism. Additionally, some cats do not like this food, and they truly cannot have any other supplemental food or treats. Restricting cats to this strict diet can be difficult in a home with multiple pets. However, dietary therapy can be a good option for cats who have side effects with other treatments or other disease processes that restrict alternative treatment options.
Overall, the prognosis associated with feline hyperthyroidism is good. This is a treatable disease, and for the vast majority of cats we can get their thyroid levels back to the desired range. Untreated, hyperthyroidism will be fatal with time. If you have concerns about your cat’s health or the management of their hyperthyroidism, please reach out to your veterinarian to discuss further care. We are here to help! “