When it comes to feline hyperthyroidism, there are four treatment options: radioactive iodine, surgery, medication, and iodine-restricted diet. Each of these approaches has unique pros and cons.
Although radioactive iodine treatment is regarded as the treatment of choice for most cats, the reality is that the optimal treatment for any individual cat is based on a number of patient and client factors.
In this treatment, cats are hospitalized at a treatment facility and injected with intravenous radioactive iodine. This iodine is quickly taken up by the thyroid gland, in order to produce T3 and T4. While concentrated in the thyroid gland, radioactive iodine destroys abnormal thyroid tissue with no damage to the surrounding tissues. This treatment also effectively eliminates extrathyroidal thyroid tissue, which is present in up to 20% of hyperthyroid cats.1
The major advantage of radioactive iodine therapy is that it can produce a complete cure with a single, one-time treatment. Thyroid levels in affected cats typically normalize within weeks and, within three months, 95% of treated cats have been cured of their hyperthyroidism.2 Additionally, this treatment does not have any serious side effects. A small percentage of cats (2-7%) may exhibit transient post-treatment hypothyroidism; some of these cats may show clinical signs and require thyroid hormone supplementation.1
The primary disadvantage of radioactive iodine is that it requires special handling, meaning that treatment can only be performed at specialty hospitals or other specially-equipped facilities. Additionally, cats must remain quarantined for several days after treatment, before they can return home to their owners. Therefore, radioactive iodine may not be a good option for cats that have significant underlying disease (such as renal disease, cardiovascular disease, etc.) that may be exacerbated by the stress of boarding.
Surgical thyroidectomy is also a curative treatment for feline hyperthyroidism. Unlike radioactive iodine, however, it requires general anesthesia. Additionally, the parathyroid glands can be easily damaged during thyroidectomy, leading to disorders in blood calcium balance. Given these drawbacks, surgical thyroidectomy is rarely recommended for hyperthyroid cats.
Methimazole and carbimazole act to reduce the production and release of thyroid hormone. Although these medications are not curative, long-term administration can effectively control the clinical signs and adverse effects of hyperthyroidism. They are effective in almost all cases of hyperthyroidism, except in the case of thyroid carcinoma.1
While medical treatment is very effective, it can have side effects and requires owners to medicate their cat twice daily. Side effects may be seen in up to 40% of cats receiving oral medical therapy and these effects may include vomiting, facial pruritus, anemia, thrombocytopenia, and hepatopathy.1 Transdermal gel may be an easier option than oral medication for some cat owners; although it may be slightly less effective than oral medication, it is often sufficient to achieve clinical control.
Because the production of thyroid hormones requires iodine, iodine-restricted diets such as Hills y/d® can also play a role in the management of hyperthyroidism. These diets restrict dietary iodine to extremely low levels (approximately 2 ppm), which inhibits the synthesis of excess thyroid hormone. Iodine restriction is an effective treatment for most cases of feline hyperthyroidism, with one study finding that 86% of cats experienced resolution of clinical signs and normalization of thyroid hormone levels.1
Dietary therapy can be a good treatment option for a cat that is not a candidate for other treatments. If a client chooses to go this route, however, it’s important to ensure that the cat is not given access to any other cat food, treats, or table food. Additionally, this therapy is only effective for indoor cats, because outdoor cats may eat rodents and other items that would eliminate the benefits of the iodine-restricted diet. Also, if the owner has other cats in the home, the hyperthyroid and non-hyperthyroid cats should be fed separately, because we do not currently know the effects of feeding an iodine-restricted diet to a normal, healthy cat.
While the veterinarian will recommend the best treatment for each individual hyperthyroid cat, it’s ultimately up to the client to determine which treatment approach is the best fit for their lifestyle and financial situation. Therefore, it’s important for you to have a basic understanding of the four treatment approaches, so that you can answer client questions as they arise and help your clients make educated decisions.
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