Q. My 12-year-old cat has hyperthyroidism. She cannot tolerate methimazole, and I have tried special food (she didnt like it), homeopathic treatments, dietary additives and even consulted an animal psychic. Nothing has an effect, and I dont want to put her through surgery or radioactive iodine therapy. They both sound horrible and also too expensive. Have we run out of options?
A. I understand your concern, and I have had many feline friends who have dealt with this common disease. Hyperthroidism is one of the most common endocrine diseases of cats, affecting up to 10 percent at some point in their lives (usually middle age to older cats).
Effective treatment options for hyperthyroidism, as you have alluded to, include methimazole or carbimazole administration, radioiodine therapy, surgical removal of the thyroid gland, and perhaps restriction of dietary iodine. Surgical removal of the thyroid gland is not very commonly performed any longer, as it involves the risks associated with anesthesia and surgery, and other effective options are available. It also may be complicated by inadvertent removal of other important tissue such as the parathyroid gland.
Dietary iodine restriction may be an effective therapeutic option for feline hyperthyroidism, but the effectiveness and long-term health effects of this mode of therapy remain controversial.
Methimazole inhibits the production of thyroid hormone, and this therapeutic option is most commonly quite effective at controlling hyperthyroidism in cats who can tolerate it. This option usually requires the oral administration of medication two to three times a day, which can be difficult for some owners. Transdermal application may be effective in some cases. In some cats, prolonged administration may result in resistance and loss of control of thyroid levels.
Carbimazole is a drug that is basically converted to methimazole after it is administered. As you have unfortunately experienced in your kitty, some cats have adverse reactions to these drugs. Some of these reactions can be relatively minor, including itching, hair loss, and an upset stomach, while others, such as decreased production of red and/or white blood cells, can be much more serious.
Radioiodine therapy involves the injection of radioactive iodine (I-131). This compound, which emits radiation only over a very small distance, is absorbed and concentrated in an affected cats thyroid gland, and the radiation emitted destroys overactive thyroid tissue, most commonly resulting in a return to normal thyroid hormone levels within two weeks of treatment.
Radioiodine therapy is approximately 95 percent effective, and it is generally very well tolerated. Cats receiving I-131 must be quarantined within a controlled facility for between three and five days, but most cats tolerate this component of therapy quite well. I-131 therapy avoids the risk of surgery and the inconvenience and side effects of daily methimazole administration, and although complications, including inappropriately low thyroid hormone levels, may occur following this treatment, they are very rare and generally manageable.
With respect to the cost of 1-131 therapy, while it may seem to be an expensive option, comparison with the cost of either surgery or long-term methimazole administration, which requires regular follow up with a veterinarian, suggest that it may not be as comparatively expensive as it seems long term.
Recently, dietary iodine restriction has been proposed as an option to treat hyperthyroidism in cats, and there is some evidence that this treatment may be effective. It is important to point out, however, that the long-term effectiveness and health effects of dietary iodine restriction are the subject of some debate, making this mode of therapy controversial at this time. Further research is necessary to establish the long-term effectiveness and safety of this mode of therapy.
I hope that this is helpful, and that this letter finds you and your kitty feeling well. I recommend that you discuss these options with her veterinarian, and stay tuned for future advances in the prevention and treatment of this common feline disease.
-Best regards, Elizabeth
Elizabeth is thankful for the assistance of Bruce G. Kornreich, DVM, Ph.D., ACVIM, Associate Director of the Cornell Feline Health Center, in providing the answer on this page.