Hyperthyroidism is a hormonal disorder in which the thyroid gland produces excessive amounts of thyroid hormone. The cause is almost always a benign tumor of the thyroid gland, but why it occurs in the first place is not known. Malignant thyroid tumors cause less than 2% of the cases. Hyperthyroidism is a common disorder in older cats. The average age of cats at the time of diagnosis is 9-10 years. The classic presentation of hyperthyroidism is an older cat that is losing weight despite having a ravenous appetite. Often the heart rate and blood pressure are increased, and there may be a heart murmur. Many things could cause these clinical signs in an older cat. Therefore, the veterinarian usually runs a panel of tests to check all body systems. The diagnosis of hyperthyroidism is confirmed by measuring thyroid hormones in the blood.

Often the initial laboratory tests have already been done by your veterinarian. The tests that are needed prior to treatment are listed in our Information for the Referring Veterinarians on our website, which you can read. Any of the pre-requisite diagnostic tests can be performed by your veterinarian or here at the MSU Veterinary Medical Center. However, the costs of those diagnostics are not included in the package price for treatment. Patients are scheduled and admitted through the Internal Medicine service. Appointments are booked on Mondays or Tuesdays so that patients are more likely to be ready to go home by the upcoming weekend.

The package price for the initial visit with the internist, the ¹³¹I treatment, and the necessary hospitalization is $1,400-$1,600 depending on the length of stay required. This does not include the cost of the pre-requisite diagnostics. It also does not include the cost of any additional diagnostics or treatments that some cats might need. If your cat needs any of them, they will be discussed with you before they are performed with your approval. You can read about some examples of additional diagnostics in the Information for Referring Veterinarians to see if they might apply to your cat. You can also discuss it with your veterinarian.

The thyroid gland normally produces hormones that are necessary to maintain normal metabolic functions throughout the body. When too much thyroid hormone is produced, the body’s entire metabolism becomes over-active, which is why it is called thyroidism. The excess thyroid hormones cause all organs to do more work than normal. This results in weight loss, increased appetite, increased thirst, high blood pressure, increased heart rate, and sometimes other problems like excitability, vomiting or diarrhea. Overall body condition suffers, but the heart and kidneys are of particular concern. Without treatment, hyperthyroidism can eventually cause the heart to fail. Excess thyroid hormones can make kidney function appear to be better than it actually is. For all these reasons, chest radiographs (X-rays), blood pressure, and blood and urine tests are performed before and after treatment.

• Medical management. The thyroid hormones are made with iodine. A medication known as interferes with the incorporation of iodine into thyroid hormones. It is a tablet given orally, twice a daily. It can also be administered as a transdermal gel, applied twice daily to the inside of the pinna of the ear. Methimazole is given for the rest of the cat’s life because hormone production returns when the drug is discontinued. Although it is effective, some cases do not respond to the usual doses. It is difficult to administer twice-daily oral medications to some cats. About 15% of cats have side effects.
• Surgical removal of benign thyroid tumors. In the hands of a qualified surgeon, this is an effective treatment that can cure hyperthyroidism. However, it is not always possible to identify all the abnormal tissue, especially if the growth is tiny or in an abnormal location (ectopic thyroid tissue), in which case, hyperthyroidism may return. Some patients, such as those with serious heart disease, are not good anesthetic or surgical candidates.
• Radioactive iodine, also known as radioiodine or ¹³¹I. This is a painless, safe and effective non-surgical means of treatment that, like surgery, is a potential cure for hyperthyroidism. The details are described below.
• After any one of these treatments, kidney function may decline. It all depends on what, if any, pre-existing kidney disease there might have been, and the effects of hyperthyroidism itself. This is why pre- and post-treatment kidney evaluations are so important.

Iodine is used by the body to make thyroid hormones, so it naturally goes to the thyroid gland instead of to other tissues in the body. When radioactive iodine is administered, it is incorporated into the thyroid hormones by the thyroid cells, just like regular iodine. The more hyperactive the thyroid cells, the more radioactive iodine accumulates in them. Enough radioactivity is emitted in the hyperactive cells to kill them. This is completely painless to the patient, who feels nothing different while this is happening. The normal thyroid cells return to normal function after the death of the hyperactive ones.

Occasionally, the radioactive iodine does not kill all of the hyperactive cells. This happens in approximately 5-10% of hyperthyroid cats treated with radioactive iodine. If it happens, hyperthyroidism may return, necessitating a second radioactive iodine treatment or one of the other treatment options. Occasionally, normal thyroid cells are also destroyed. If that happens, thyroid hormone replacement can be given as a daily pill.

We administer the radioactive iodine subcutaneously (under the skin), between the shoulder blades. It is no more difficult or painful than a typical vaccination. There is rarely any tenderness or swelling at the injection site.

The radioactive iodine emits radioactivity. That is how it works to kill the hyperactive thyroid cells. The¹³¹I has accumulated in the cat’s thyroid gland and will be eliminated from the body mainly through the kidneys in the urine, with lesser amounts through the feces. Anything that comes in contact with the cat or its urine or feces is exposed to radioactivity, until it is has been entirely eliminated from the body and the radioactivity itself decays. Items such as litter, food bowls, bedding, toys, collars, carriers and most importantly, people, are exposed. To maintain radiation safety, there is a special ¹³¹I ward and only specially trained personnel can work with the patients. This is why owners cannot visit with their cats and why none of the patient’s “personal” items can go home again. If you and the attending clinician agree that a “personal” item, like a favorite toy, can be left with the patient, you must also agree that it will not be released when the patient goes home.

The amount of time needed for your cat’s body to eliminate the ¹³¹I depends on how much was bound to the thyroid tissue, how well your cat’s kidneys are functioning, and how much urine and feces are being passed. Typically, this is 3-5 days, but on occasion may take up to 10 days. Your cat and the environment will be monitored everyday with a Geiger counter or survey meter to determine how much radioactivity is left.

Twice daily your cat will be monitored and cared for by our personnel who have been specially trained for handling radioactive substances. This entails the usual things such as checking the cat’s attitude, appetite, and condition, and feeding, watering, litter change, and cage clean up. To minimize exposure of our personnel, non-critical medications (for example, once-monthly flea control) are not given during this time. There are strict radiation safety guidelines that must be met before your cat can leave the ¹³¹I ward and go home. Each day the cat’s body and its litter will be monitored with a Geiger counter or survey meter to determine if the radioactivity is below the level determined to be safe by the University, the State of Michigan and the Nuclear Regulatory Commission (NRC). (See the accompanying photo of Bubba. The survey meter is simply passed over the cat’s body.) When radioactivity has reached the safe level, your cat can go home.

The radioiodine is eliminated in urine and feces. By 14 days after treatment, the radioactivity will have decayed to a very low level. Until then, the radiation safety guidelines of Michigan and the NCR are followed. Pregnant women and young children must not handle the cat or clean the litter pan. Older children under the age of 18 should limit their contact with the cat and should not clean the litter pan. For these 14 days everyone should avoid sleeping with the cat or holding it close for periods of time, such as lying in your lap while you watch TV. Always wash your hands after handling your cat or the litter pan. This will help limit exposure to you, to others, and contamination of other areas of your house. By 14 days after treatment radioactivity is essentially gone and you can safely handle your cat and the litter as usual.

Special care should be taken with the litter box for 14 days after treatment to avoid environmental contamination with radioactivity. We recommend using clumping litter, cleaning the litter box at least once daily and wearing disposable gloves. If you believe that the litter itself will not harm your plumbing or sewage system, it is acceptable to flush the clumps and feces from the litter box down the toilet. Alternatively, the used litter and disposable gloves can be collected in plastic bags and set aside (for example, in an out-of-the-way corner of the garage or basement) for 14 days. By 14 days after treatment the radioactivity will have decayed and it is safe for people and for the environment to dispose of the litter as you normally would. Dealing with the used litter will be explained to you when you take your cat home.

Your cat’s thyroid and kidney functions should be tested at 1 and 3 months after treatment. In some instances, kidney disease that was not evident prior treatment may develop. Blood and urine tests are the only way this can be detected. The amount of thyroid hormone left after treatment is also determined with blood tests. The post-treatment monitoring can be done by your veterinarian or by the Internal Medicine service at the MSU Veterinary Medical Center, 517-353-5420 (ask to speak to the internist on consult calls).