Hyperthyroidism is a disease of older cats (average age is 12-14 yrs old) caused by excessive secretion of thyroid hormone by abnormal thyroid cells located in a (usually) benign nodule in one or both thyroid glands (1% of hyperthyroid cats can have a malignant thyroid cancer for which 131I is the best treatment). Thyroid glands are located in the neck. Elevated thyroid hormone levels cause increased metabolism and the other common clinical signs of hyperthyroidism: weight loss, appetite changes, increased water consumption, vomiting, diarrhea, rapid heart rate, heart murmur, elevated blood pressure, increased vocalization, muscle weakness and poor haircoat, although not every cat experiences all of these symptoms. Hyperthyroidism tends to be a gradual onset disorder, with many cats losing weight slowly over several months or a few years.
If left untreated, the increased metabolism will eventually cause organ failure (heart, kidney, blindness due to retinal detachment) and premature death. We do not currently know why hyperthyroidism is a common disorder in older cats.
Your veterinarian can perform blood tests to determine if your cat has hyperthyroidism. Since older cats with the most common signs of hyperthyroidism (weight loss, appetite changes and vomiting) can also have other problems such as kidney disease, heart disease, diabetes and inflammatory bowel disease, a thorough physical examination, blood testing and urinalysis are usually indicated to allow diagnosis of hyperthyroidism. Radiographs, cardiac ultrasonography and blood pressure determination are also sometimes performed.
There are four options for treating feline hyperthyroidism: medical management, dietary management, surgery to remove the thyroid gland(s) and radioactive iodine (131I) therapy.
Oral or transdermal antithyroid medications (methimazole) can be used to reduce excessive thyroid hormone secretion. These medications must be given for the rest of the cat's lifetime. Side effects serious enough to warrant discontinuing using these medicines occur in up to 5 percent of cats taking them and can include loss of appetite, vomiting, liver dysfunction, bone marrow suppression and allergic skin reactions. Hyperthyroid cats being maintained on antithyroid medicines should have regular blood testing (every 4-6 months) for the remainder of their lifetime. A common misconception about medical management of hyperthyroidism is that it's the least expensive way to treat the disease. Antithyroid medicines can cost up to $2.00 per day, depending on which formulation is used. Also, the periodic examinations and blood testing required two or three times yearly to accurately and safely dose these medications can cost about $150 to $350 per year. This means an owner will spend $450.00 to $850.00 per year for the rest of the cat's life to medically manage hyperthyroidism!
An iodine restricted diet (Prescription Diet Y/D) can be used to control hyperthyroidism. Cats must eat ONLY the Y/D diet for this treatment option to succeed.
Surgery to remove the thyroid gland(s) involves general anesthesia risk and can result in serious calcium regulation difficulties after surgery if the parathyroid glands, or their blood supply, are inadvertently damaged during surgery. Following successful surgical treatment for hyperthyroidism, 10-20 percent of cats will become hyperthyroid again in their lifetime. Surgery can be the treatment of choice in the rare hyperthyroid cat with severe medically unresponsive and life threatening congestive heart failure because one can achieve resolution of hyperthyroidism in a matter of hours. Cats with very large thyroid nodules (more than 3-4 cm diameter) can also require surgery to debulk their thyroid masses before treatment with 131I.
Radioactive Iodine (131I) Therapy:
Radioactive iodine (131I) therapy takes advantage of the fact that dietary iodine is used in the body only by cells of the thyroid glands. 131I therapy uses a radioactive isotope of iodine that is administered as an injection under the skin (just like vaccines are given). The 131I will be taken up preferentially into the "most active" thyroid cells, which are the thyroid cells in the nodule(s) or tumor(s) that are overproducing thyroid hormone. The radiation energy is emitted and absorbed within a few millimeters of where the iodine accumulates in the body (i.e. the hyperthyroid tissue) and will kill these abnormal thyroid cells.
Anesthesia is not required for 131I therapy. Following 131I administration, cats are housed in individually ventilated large cages in a shielded radiation safety room for 3 nights to allow most of the radioactive iodine to be excreted and remain behind at the clinic. The 3 night stay after 131I administration is required by Oregon Radiation Protection Statutes.
The first dose of 131I will cure hyperthyroidism in 97-98% of hyperthyroid cats. Follow up thyroid testing should ne performed by the referring veterinarian 4-6 weeks after treatment. If a second dose of 131I is required within the first six months to cure your cat, there is no charge for this procedure provided the follow up labwork is performed in a timely manner. Following successful 131I therapy, only 1 in 200 cats will become hyperthyroid again in their lifetime.
The initial dose is usually 1-6 mCi depending on the cat's thyroid blood test result within 90 days of the treatment date, careful physical examination of the thyroid gland(s), the duration of the hyperthyroidism, body weight, age and kidney function.
Current radiation safety rules require your cat to be hospitalized a total of 4 days.
Each cat is housed in a large ventilated cage in the radiation ward with plenty of soft bedding and a litterbox. Fresh food and water are always available and the litter boxes are checked several times each day and cleaned when necessary. We prefer to feed cats exactly what they are used to eating at home during their stay here. If you wish to bring treats, please do so. Most cats seem to prefer having a familiar item from home such as a toy, a bed or some clothing with them during their stay. You are encouraged to bring such items, but they cannot be returned to you when your cat goes home, so please don't bring anything you'd be heartbroken not to get back. Radiation Protection statutes prohibit visitors in the restricted area of the clinic where the cats are housed. Dr. Evans will call you daily to provide a progress report on your cat and to answer any questions you have.
To be candidates for 131I therapy, a physical examination and screening lab testing (CBC, chem panel, T4, and a urinalysis) must be performed by the referring veterinarian within 90 days of the treatment date with the cat off of any methimazole for at least 7 days.
A "methimazole trial" should be performed prior to referral for 131I therapy if your cat has evidence of IRIS Stage 2-4 renal failure. The average age of cats treated here is 14 years old. Age is not a contraindication for 131I therapy if your cat is in otherwise good health.
There are no direct side effects from 131I therapy. As the hyperthyroidism is resolving many cats will be somewhat quiet or sleepy for the first few weeks. As mentioned before, resolutions of hyperthyroidism can un-mask pre-existing kidney disease. Up to 5-10% of treated cats may require daily thyroid supplementation following 131I therapy.
Your cat will be temporarily and slightly radioactive when discharged home to your care, although 90-95% of the administered 131I will have been excreted by then. Radiation safety precautions must be observed for approximately 2 weeks after discharge to comply with state rules. During this 2 week period of time, you should limit close contact with your cat (usually you're allowed up to one hour of contact per day closer than one foot to your kitty), keep your cat more than a few feet away from children and pregnant women at all times, and you must not dispose soiled litter in the trash to comply with state rules. Soiled litter can be collected, stored in a secure location on premises for 10-12 weeks, and then disposed of in the trash once all radioactivity has dissipated. Alternatively, soiled litter and feces can be flushed down the toilet rather than be thrown out with the trash for the two weeks after discharge if flushable litter is used. Litter boxes need to be kept away from small children. We are not allowed to put soiled litter into the trash as it can set off radiation detectors at solid waste handling facilities resulting in significant fines.
The amount of radioactivity remaining in your cat at discharge is quite small. If your cat were a person undergoing 131I therapy, he or she would have received 10 to 15 times as much 131I and been able to go home the same day of treatment with fewer restrictions than your cat has. The goal of the precautions you must observe is to keep radiation exposures for members of the general public as low as reasonably achievable.
There are no restrictions insofar as other pets are concerned; you don't need to quarantine your cat from other pets. There is no hazard to people or other pets if the precautions are followed.
Your cat's thyroid level (T4) should be checked by your regular veterinarian 4-6 weeks after treatment. T4 levels should normalize within this time, although a few cats can take as long as 6 months to become normal. If the T4 level is still high at the 4-6 week recheck, we will discuss a plan for further monitoring. Most owners notice improvement in their cats within the first month following 131I therapy: weight gain, normalization of food and water intake and a reduction in other hyperthyroid symptoms like vomiting, diarrhea and poor haircoat.
The fee for 131I therapy is $1700 to $2000 depending on the dose of radioiodine administered (May 2021). This includes consultation before and after treatment, the admission and discharge office calls, initial physical examination, radioactive iodine, hospitalization and nursing care, monitoring and radioactive waste disposal. If your cat requires a second treatment because the first treatment did not completely resolve the hyperthyroidism within the first few months, there is no charge for the subsequent treatment, provided that all required follow up labwork was performed in a timely manner. Payment is due upon discharge from the clinic. We accept cash, personal checks, VISA, MasterCard, and Discover. For clients interested in payment plans, we do accept Care Credit. More information about this option is available at www.carecredit.com.
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