Feline Hyperthyroidism

Hyperthyroidism is a commonly diagnosed endocrine disease in older cats. It is caused by a tumor that produces too much thyroid hormone. The disease is eventually fatal if left untreated.

The treatment of choice for cats with feline hyperthyroidism is a single injection with Radioactive Iodine (I-131), just as it is in humans. Radioiodine is safe and effective with cure rates up to 95 - 98% with one injection. Cats can receive a second treatment, if necessary. A single injection is given under the skin, like a vaccine. The radioactive iodine is quickly absorbed into the bloodstream where the iodine is taken up by the thyroid gland. Once in the thyroid, it destroys the overactive portions of the thyroid gland but spares the healthy tissue.

The majority of cats will achieve normal thyroid levels within 1month of treatment but some can take up to 3 months. Hospitalization is required. A short stay (1-2 days) in the AIM hyperthyroidism treatment facility is required by law until the level of radiation is at an acceptable level. You can expect that your cat will remain at the clinic for 1 to 2 days after injection. Additional minor precautions after discharge are needed but they are not difficult and will be thoroughly explained to you.

Advantages of I-131 treatment over medical and surgical treatments for feline hyperthyroidism:

• No anesthesia is required

• No daily medication

• Does not destroy healthy tissue

• Does not damage other tissue or organs, including the parathyroid glands

• Does not have harmful side effects

• Returns thyroid function to normal usually within one month

• Is cost effective- The cost of radioactive iodine therapy is similar to the other forms of therapy, and is often less expensive when compared to long term medical therapy or the higher probability of a required second surgery.

Other ways to treat feline hyperthyroidism:

Medical therapy- (methimazole - felimazole - Tapazole®) is required daily, is a life-long treatment, and does not cure the cat's hyperthyroid condition. The difficulties associated with giving pills to cats often causes stress to both the animal and owner. Tapazole may have side effects, particularly to the liver and blood system. The drug therapy must be adjusted, based on periodic blood tests, and the cost of treatment and monitoring tests is usually over $800-1000 per year for the life of the cat.

Surgical therapy for feline hyperthyroidism requires anesthesia, may need to be repeated, and may result in a deficiency of parathyroid hormone if these glands are inadvertently removed. Surgery may not be effective in removing all of the affected thyroid tissue causing feline hyperthyroidism.

Hill’s y/d diet- The cat may have no access to any other food (no human foods, cat treats, other cats food, etc) or it will be rendered ineffective. Many cats find this diet unpalatable.

PRE-TREATMENT Recommendations:

Cats who are on oral treatment (Methimazole) should stop taking the medication 7-10 days prior to their treatment date unless they have significant heart disease. Patients who are on Hill’s y/d food will need to be off that food and eating a non-prescription diet 14 days prior to their treatment date.

What to expect during your appointment

Our consultation will review past history and laboratory results; we will conduct additional physical and laboratory examinations to confirm the initial diagnosis and make sure that no other problems exist that would complicate treatment. All cats are screened closely for the presence of other illnesses, especially heart and kidney disease. The usual screening tests include a complete blood count, chemistry, Felv/FIV screening, urinalysis, and X-rays/ultrasound. Your primary care veterinarian may perform the routine tests prior to the final evaluation. All tests and procedures and the related costs will be explained; you can accept or decline recommendations during these discussions. We strive to be thorough and recommend those procedures that are essential to your cat and avoid unnecessary testing.

• If radioiodine is the treatment of choice, we will explain the procedure and keep you informed during hospitalization (1-2 days).

• If appropriate, a methimazole - felimazole (Tapazole®) trial should be conducted to assess the potential for unmasking renal disease that may have been “hidden” by the hyperthyroid state.

• This trial is performed by giving methimazole - felimazole (Tapazole®) at an adequate dose to induce a euthyroid state for at least two (2) weeks – this is determined by monitoring the T4 value.

• Once the pet has been euthyroid for two weeks while on medication, then kidney function is assessed by evaluating the BUN, creatinine, SDMA and urine specific gravity values.

• If the laboratory findings indicate stable kidney function in the euthyroid pet while being given methimazole - felimazole, then there is a greater probability the patient will have stable renal function following I-131 therapy.

• Methimazole - felimazole therapy should be stopped at least 7-10 days prior to I-131 therapy.

Hospitalization after injection

Each cat is monitored, fed, given fresh water and their activities are recorded on an ongoing basis every day. Once your cat has been injected with the I-131, the amount of interaction between the pet and support staff is limited by law for the radiation safety of staff members. This is why your cat cannot be injected and immediately sent home. These patients are basically “radioactive”. As the radioactivity becomes concentrated in the thyroid gland, it kills the abnormal thyroid cells. It is this eventual selective concentration of the I-131 in the thyroid gland that makes this therapy so effective (>95% cure with one injection). It is this relatively high level of radioactivity that requires limited interaction with the pet during their stay in the AIM facility. They cannot be removed from their cages to be cuddled, played with, or allowed to roam freely in the treatment rooms. This would be inappropriate and dangerous for those managing the care of your pet.

If there is a health concern, the overseeing specialist will take the necessary actions to assure appropriate health care is given.

Guidelines for cat owners after radioiodine therapy

Cats treated with radioiodine (I-131) have a low level of radioactivity at the time of discharge from the clinic. This level is sufficiently low that complete isolation of the cat from people is not required. Follow these guidelines for 3 weeks after your cat comes home to eliminate or further reduce human exposure to radiation that is emitted by the cat and by radioiodine that is excreted by the cat in urine and saliva.

• Do not allow your cat to sit on or sleep with anyone after coming home. Spend as little time as possible close to your cat. Casual contact is fine, but prolonged lap sitting and sleeping in your bed should be avoided. You can relax these guidelines for adults after week 2, but not for young children or women who are pregnant or might become pregnant.

• As an extra safety precaution for the very young and developing fetuses, children under 10-years-old and pregnant women should avoid contact with your cat, food dishes, toys and litter for the full 3 weeks.

• Keep your cat separate from any other cats in the household in order to reduce the risk of radiation exposure. Please ensure that your other cats do not use the same litter box, as radioiodine is excreted in the urine. Please ensure that your other cats are fed from separate food bowls and water dishes as radioiodine is also excreted in the saliva.

• Wash your hands with soap and water after touching your cat or anything the cat has been in contact with (e.g., food dishes, toys, bedding, and especially the litter box). When cats groom themselves saliva contaminates their hair coat, which can then contaminate dishes and other objects.

• If possible, use flushable litter for the next 3 weeks. Otherwise use scoopable litter and collect all waste in a sturdy container lined with two plastic bags (one placed inside the other). Store waste in a well-ventilated space away from your primary living areas, and hold for an additional 2 weeks so natural decay will reduce radioactivity to background levels. Then the litter may be disposed with the normal trash. Landfills do not allow the disposal of low-level radioactive waste and are equipped with sensitive radiation detectors. You may be charged over $1000 if radioactivity is detected in your cat's litter at the landfill.

• Please use extra care when cleaning the litter box to avoid getting soiled litter on your hands. Use disposable plastic gloves and litter box liners to help prevent contamination. Wash with soap and water after cleaning the litter box. All used disposable gloves should be stored with the waste—treat them as if they are contaminated.

• Please keep your cat indoors if the cat uses garden areas as a litter box. Cats may go outside for supervised exercise.

• Do not allow your cat to eat from your plate or walk on counter tops where food is prepared. If the cat does or you are not sure, then put on disposable gloves and thoroughly wash the area with soap and water before preparing food.

• Items that your cat routinely contacts (e.g., bedding, toys) should be thoroughly washed with soap and water prior to handling them without disposable gloves.

• If your cat salivates on hard surfaces, then these areas should be cleaned often with soap and water, or a spray cleaner (e.g., Formula409). For surfaces that are difficult to clean (e.g., couches and upholstered chairs) cover these surfaces with a towel or blanket and replace as necessary. These towels and blankets should be washed separately from your clothing using standard laundry detergents.

• If your cat vomits a hair ball, put on disposable plastic gloves and dispose of the material with the waste litter or flush down the toilet. Next, clean the area with soap and water or spray cleaners.

• If your cat needs emergency care within the first 3 weeks of coming home, then advise the veterinary staff that your cat has recently undergone radioiodine therapy.

• After a month (typically after your first recheck appointment with your veterinarian) you can resume all normal routines and assume that all hazards associated with radioiodine treatment are gone.

FAQ’s