There are three treatment options available to treat hyperthyroidism. We prefer to think of them as good (medical management), better (surgical removal of affected thyroid tissue) and best (I-131 treatment).
All have their pros and cons, which are described below. It is important to note that an article in the Journal of the American Veterinary Medical Association (Feb. 15, 2006) showed that cats treated with I-131 lived more than twice as long as cats treated with medical management (methimazole) alone.
Surgery has long been a very popular treatment option for hyperthyroidism. It includes surgical removal (thyroidectomy) of diseased thyroid tissue. Surgery has the benefits of decreasing thyroid hormone rapidly and is often the treatment of choice when it is imperative to do so, such as with severe heart failure caused or exacerbated by hyperthyroidism.
However, surgery presents significant risks to the patient. First and foremost, surgery requires anesthesia of a geriatric patient who typically has compromised renal function and/or heart disease. Also, the parathyroid glands, which help maintain normal calcium and phosphorus levels, are located on both ends of the thyroid glands. During surgery these can be destroyed or disrupted, resulting in a life-threatening change in calcium and phosphorus levels and require support short term and rarely life-long. Also, cats with bilateral thyroidectomy may require thyroid hormone supplementation. Additionally, cats may hide some hyperthyroid tissue within the chest and cannot be detected unless advance imaging techniques are used for the identification and removal.
Due to the risks of anesthesia and post-surgical calcium metabolism problems, surgery for this disease is generally not recommended if I-131 is available.
Medical therapy is through the administration of methimazole or PTU either orally or transdermally (surface of the skin). The medication manages the disease but when withdrawn, hyperthyroidism redevelops. Unless a malignancy is considered likely, methimazole is usually the first treatment choice of hyperthyroid cats. Repeat bloodwork(s) are required to track change in thyroid levels as well as kidney function and side effects of the medication. The medication may seem like the cheapest solution, at least in the short term, but is often more expensive in the long run as hyperthyroidism requires life-long therapy and hyperthyroid cats can live for years. After the initiating period, blood checks every 6 to 12 months are required for monitoring as the methimazole needs usually increase as the thyroid tissue increases in hyperactivity. The downsides of this treatment include the fact that cats will require medication from 1-3 times/day and up to 20% may also suffer from side effects. The most common side effects include inappetance, vomiting and diarrhea but a few may develop a dramatic allergic reaction which leads to self-inflicted wounds about the face/ears. In addition, medicine can cost up to $2 per day. After adding the cost of medication, blood work monitoring and the personal difficulty of medicating, especially with some cats, the cost can be high especially after a year or better of treatment. And this doesn't even figure in the possibility of side effects that may require medical attention.
That said, medical management may be the best choice in cats with concurrent life threatening diseases. Because we are often dealing with an older population, our patients often have concurrent moderate to severe kidney disease, heart disease or cancer and the medication may benefit these patients.
I-131, also known as radioactive iodine, is the treatment of choice for most cases of hyperthyroidism in cats. Hyperthyroid cats have both normal thyroid tissue and abnormal hyperactive thyroid tissue and only active thyroid tissue will take up iodine. A negative feedback system in the body regulates the normal thyroid production and since thyroid level has been elevated, the normal tissue should not be functioning. Thus, when I-131 is injected, the treatment is strategically targeted at the hyperactive thyroid tissue and ablates it. Ideally the normal thyroid tissue escapes unscathed and will return to normal function after the thyroid levels drop and stimulate production of thyroid hormone again. It is generally considered the safest and most effective therapy available. With the exception of hyperthyroidism due to thyroid cancer (thyroid adenocarcinoma) I-131 has an approximately 95% cure rate after a single injection and a purported 100% cure rate after a second injection. At The Cat's Meow Feline Veterinary Clinic, cats requiring a second injection will be retreated at no charge unless treating a carcinoma. A very small percentage of cats treated with I-131 will become hypothyroid (0.25%) but even fewer require thyroid supplementation.
Side effects of I-131 are rare and include mild lethargy and decreased appetite. Occasionally a sore throat and/or voice change can occur and are usually transient. Although significant side effects in cats are rare, if your cat exhibits other symptoms besides mild lethargy / decreased appetite do not assume this is normal. Because we are dealing with a geriatric population, other underlying health concerns may arise and your veterinarian should be consulted immediately.
Although humans receiving similar treatment are usually allowed to go home following Iodine 131 treatment, the process in cats is controlled by the Nuclear Regulatory Commission (NRC). Because of these regulations, your cat will be required to remain hospitalized in a special isolation ward until the radiation is at sufficiently low levels. Most cats are in isolation for 3-4 days until radiation levels have dropped enough to meet these regulations. Also, after the cat comes home some restrictions are imposed to limit your overall exposure to radiation: the cat must use special flushable litter, the cat is not allowed outside, the cat's daily direct contact time with the owner is limited, children and pregnant women may not have contact with the cat during this time, etc.
The Cat's Meow Feline Veterinary Clinic and Hyperthyroid Treatment Center
1017 South Perry Street
Spokane, WA 99202
(7:30 drop off)