Hyperthyroidism in cats can be treated in several different ways. Medical management is possible and is often effective in controlling clinical symptoms but does not cure the disease. Surgery to remove the diseased thyroid glands is possible and has the advantage of being curative but has some disadvantages as well. Radiotherapy is generally considered to be the safest and most effective method of treatment for feline hyperthyroidism but it too has advantages and disadvantages.
Medical Management of Feline Hyperthyroidism with Methimazole
Methimazole is the drug of choice for hyperthyroidism in the United States. In Australia and the United Kingdom, carbimazole is often used instead of methimazole because this drug is easier to obtain in these locations. Carbimazole is metabolized to methimazole within the body so the effects of the two drugs are quite similar.
There are advantages to using methimazole (or carbimazole) to treat a hyperthyroid cat.
- Methimazole is usually quite effective in controlling the symptoms of hyperthyroidism in cats.
- Cats being treated with methimazole do not generally need to be hospitalized.
- When side effects occur, methimazole can be discontinued (or the dose lowered) and the side effects frequently resolve.
However, there are also disadvantages to using methimazole to treat feline hyperthyroidism.
- Methimazole requires daily or sometimes twice daily dosing, which may be difficult with uncooperative patients. This may be overcome to some extent by having the medication formulated into a flavored base which is more palatable for the cat or using a transdermal gel or patch which distributes the medication across the skin.
- Methimazole does not cure hyperthyroidism and must be continued for the lifetime of the cat.
- Periodic monitoring of blood tests must be performed and should include total t4, a white blood cell count, liver enzymes and kidney function tests.
Generally, medical treatment of feline hyperthyroidism is encouraged prior to pursuing other curative methods of treatment. In some cases, the changes in metabolism inherent in hyperthyroidism can mask kidney disease which may become symptomatic once hyperthyroidism is successfully treated. Cats suffering from chronic kidney disease need to be evaluated relative to their suitability for alternative curative methods of treatment which may ultimately produce a patient whose kidney disease may become life-threatening.
Radiotherapy Using Radioactive Iodine for Treatment of Hyperthyroidism in Cats
Radiotherapy involves the injection of a radioactive source of iodine known as I131 which is effective in the ablation or destruction of diseased thyroid tissue. The effects of radiation treatment are localized and the rest of the body is spared from radiation poisoning.
Advantages of radioactive iodine treatment for feline hyperthyrodism include:
- treatment is curative and no ongoing treatment is necessary. (Very rarely, hyperthyroid cats may need a second radioactive iodine treatment for complete resolution.)
- no need for anesthesia
Radioactive iodine treatments are performed only in specialized hospitals which have nuclear medicine wards. Cats must be hospitalized, usually for a period of 3-5 days following the initial radioactive iodine injection, and owners are not allowed to visit their pet or remove the pet from the hospital prior to the discharge time, persuant to regulations enforced by the Nuclear Regulatory Commission (NRC).
When discharged from the hospital, small amounts of radioactive waste may still be passed in the cat's urine. The NRC requires the use of specialized flushable cat litter for 1-2 weeks following discharge. In addition, the cat must remain indoors and contact with members of the household may need to be limited, especially in the case of children or pregnant women where contact will likely be prohibited.
Follow-up blood testing is usually recommended in the post-treatment period. Cats are generally monitored for approximately 3 months after the treatment. In rare cases, hypothyroidism may occur and may necessitate oral thyroid supplementation on a daily basis.
Surgical Treatment of Hyperthyroid Cats
Surgical treatment of hyperthyroidism involves the removal of the diseased thyroid tissue (thyroidectomy). Surgery does not require specialized facilities and it may be easier to locate a facility which is equipped to perform a thyroidectomy than to locate a nuclear medicine facility in some areas. However, thyroidectomy is generally considered riskier than radioactive iodine treatment.
- Thyroidectomy may not remove all diseased thyroid tissue, resulting in a return of the symptoms within a few months of the surgery.
- Nerve damage is possible during the surgery due to the proximity of several nerves near the thyroid gland.
- The parathyroid gland is located in close proximity to the thyroid glands as well and may be damaged or removed during the surgery. Hypoparathyroidism results in abnormal calcium metabolism, leading to seizures and other life-threatening symptoms due to dangerously low calcium levels. Calcium supplementation may be needed on either a temporary or permanent basis.
- Hypothyroidism can also occur due to surgical removal of the thyroid gland and oral supplementation of thyroid hormones may be necessary on a daily basis following the surgery.
- Hyperthyroid cats can be poor surgical candidates due to heart disease or other age-related health issues.
Treatment Options for Feline Hyperthyroidism
Treatment options for feline hyperthyroidism include medical management with drugs such as methimazole, surgical removal of diseased thyroid tissue or radiotherapy using radioactive iodine to ablate diseased thyroid tissue. Both surgery and radiotherapy are curative whereas medical management requires lifelong treatment. Advantages and disadvantages of each option must be considered carefully with respect to the individual patient's overall medical condition and the pet owner's expectations.
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