Feline Hyperthyroidism



Hyperthyroidism is the abnormal secretion of excess thyroid hormone.  In the cat this is usually due to a growth of the thyroid glands that is not cancerous.  The cat thyroid disease is not related to the common causes of hyperthyroidism in people (Hashimoto’s or Grave’s thyroiditis . Rather, it is a syndrome in which some (but usually not all) of the thyroid cells are no longer controlled by the normal feedback mechanisms that the body uses to regulate normal metabolic function.  These abnormal thyroid gland cells secrete too much thyroid hormone.  The goal of radioiodine therapy is to kill these cells and allow the normal cells to return to normal function.  This occurs in a very high percentage of cases.  Fortunately, thyroid cancer is very rarely the cause of feline hyperthyroidism.



Hyperthyroidism can be treated medically, surgically or with radioiodine.  Medical treatment consists of administration of methimazole (tapazole) 2 or 3 times per day for the life of your cat.  Typically this is administered as a tablet.  It can be prepared into a cream that is spread into the ear daily or an oil put in the food – but the dosage received by your pet typically has a hight degree of variability in the last 2 methods due to variations in absorption through the skin or variations in the amount of the food treated that your cat eats.  Surgery consists of the removal of one or both lobes of the thyroid gland.  Radioiodine treatment consists of a single (1) injection of radioiodine under the skin.



Medical therapy is effective for a lot of cats but there are several reasons it may not be the best choice.  First, some cats are simply impossible to medicate.  Second, some cats will have significant reactions to medication (eg. Blood, skin or liver problems).  Finally, some owners prefer to not have to medicate their cat(s) for a lifetime.

Surgery can present several problems: 

1.  Many hyperthyroid cats have heart problems and are not the best patients for anesthesia, and

2. If both lobes of the thyroid gland are removed at once there is a risk that there will be temporary damage to the associated parathyroid glands resulting in temporary hypocalcemia (low blood calcium).  This complication can be life threatening.  A few cats will have thyroid tissue in unusual sites (ectopic thyroid tissue) which may not be surgically accessible.  Finally, there is a risk that the hyperthyroid condition will reoccur.

Radioiodine therapy has many advantages over the other methods of treatment.  They include:

1.  Eliminating the need for you to medicate your cat for its lifetime,  

2.  Eliminating the chance that your cat will develop a problem with an antithyroid medication, 

3.  Eliminating the need for anesthesia during surgical treatment as well as the possibility that abnormal thyroid tissue might be located in unusual locations, 

4.  Eliminating any risk of low blood calcium after  surgery and 

5.  May be less expensive (assuming your cat will survive for some years with medical therapy).  The major draw back to radioiodine therapy is that it requires a brief period of isolation from your family.  There is a minimal risk (less than 1%) that the condition will reoccur after radioiodine therapy. 



Iodine is an element required for normal health.  In the cat’s body it is used almost exclusively by the thyroid glands (located in the neck) to produce thyroid hormone.  The major effect of thyroid hormone is to regulate metabolic rate.  When present in excess, thyroid hormone is toxic to several organs including the heart and liver.  Untreated, hyperthyroidism may lead to heart failure and ultimately to death.



The short answer is radioiodine kills the abnormal thyroid cells and spares the normal cells!  In cats with hyperthyroidism, there are almost always some normal and abnormal cells.  By sparing the normal cells, your cat will probably (better than 98% chance) return to his/her normal state of health and not require any further medications to control the thyroid status.



In normal cats, thyroid hormone secretion is stimulated or inhibited by the parts of the brain that recognize if the level of thyroid hormone is sufficient.  If insufficient, the brain secretes hormones that tell the thyroid glands to secrete more thyroid hormone.  In hyperthyroid cats there is a group of abnormal thyroid cells (that no longer respond to the brains normal signals) and that secrete thyroid hormone independently.  In hyperthyroid cats (unlike most hyperthyroid people) there usually is also a group of normal cells.  These are usually inactive due to the inhibition caused by the hyperthyroid state. 

Radioiodine (I-131) is a form of iodine that has been made radioactive.  In its radioactive state it undergoes a natural process of decay in which it gives off radiation and becomes an inert (non-radioactive) element, xenon. The radiation given off consists of 2 types: gamma rays (similar to x-rays) and beta particles (electrons). The gamma rays readily penetrate through the body and require special care for your pet to prevent radiation exposure to owners and hospital personnel.  These rays have little effect on the thyroid glands. However, the beta particles travel only hundredths of a millimeter in distance and is very effective at killing of the abnormal thyroid cells.  Since they travel such a small distance, they do very little damage to adjacent normal cells.  The physical half-life of I-131 is 8.1 days; that is one-half of the radioiodine converts to xenon every 8.1 days.

When injected into the body, radioiodine is accumulated into the thyroid gland.  The remainder of the radioiodine is excreted primarily in the urine and to a lesser degree the feces.  That which is concentrated in the thyroid gland emits radiation. The radiation kills the abnormal thyroid cells and treats the hyperthyroid condition.  The normal cells (since they do not take in any I-131) are spared.



Reactions to radioiodine are extremely low (no reactions to the injection have been reported). 



One possible concern is related to the fact that many cats with hyperthyroidism are older.  The old age organ of failure in the cat appears to be the kidneys.  In people, our heart wears out.  In cats, it appears their kidneys wear out.  Since we require blood testing prior to radioiodine therapy we should be able to evaluate the kidney function prior to treatment, so why would this be a concern?  The answer is that hyperthyroidism can make the kidney function blood tests appear to be normal even if there is significant disease present.  This is due to a process called hyperfiltration induced by hyperthyroidism.  The ultimate concern is that any treatment that results in the thyroid function returning to normal may expose an underlying kidney disease and it may progress or result in kidney failure.  This is a classic rock/hard place problem.  It is true that chronic hyperthyroidism probably damages kidneys (secondary to the high blood pressure frequently associated with hyperthyroidism) and therefore the kidneys are helped by a return to a normal thyroid status.  However, a rare cat could go into severe renal failure due to the fact that the  return to normal or low thyroid values “unmasked” their underlying renal disease, and this could lead to death. This occurs only rarely.



The radioiodine is administered by an injection under the skin (similar to a vaccine).  Your cat is in a room in which there are only other hyperthyroid cats (no dogs or other sick animals).  We encourage you to bring small toys or perhaps a T shirt that you have worn to bring the smell of home.  Unfortunately, since the items will probably be contaminated with radioiodine, we cannot return them to you.  Your cat cannot see any of  the other cats (although we think they pass notes to each other at night J).    Your pet is  monitored over the next few days until her/his radioactivity level is low enough to permit his/her return to you.



Bring your cat and all of its medications to the hospital.  Your primary care veterinarian will have performed the tests necessary before treatment (this includes a blood cell count, and chemistry panel).  We also recommend a urinanalysis and chest x-rays but they are not absolutely necessary.    If further tests are required they can usually be performed that day and, if necessary, treatment scheduled for another day soon.

If your cat has been on antithyroid medication {methimazole (tapazole)}  he/she must be off this medication for at least 1 week prior to treatment.  If he/she is on heart medication this should be continued through the morning of therapy and the medication should be brought to our hospital.



The doctor will meet with you and examine your pet to be as sure as possible that your pet is a good candidate for radioiodine.  He will perform the injection and he and his staff of well trained veterinary technicians will give your pet the best of care.  You will be called twice daily to update you on how your pet is fairing (almost all do very well).  You will be notified a day ahead of time about picking up your pet and scheduling its discharge.  Unlike some facilities, we do not routinely schedule patients when the doctor is not going to be present during your cats stay.



The cost of treatment for feline hyperthyroidism is significant.  The fee includes the radioiodine, the hospitalization in isolation and monitoring.  Tests required prior to and after therapy are not included as they will be performed by your referring veterinarian. These fees compare reasonably with the usual cost for surgical removal and is less than the costs for prolonged medical care.



Payment for consultation and tests must be made at the time of service.  Payment for radioiodine therapy requires a deposit of one-half the bill on admission with the remainder due upon your cat's discharge. 
If you desire, you may apply for credit through a commercial service known as CareCredit.  We have the forms and can help you apply.  If accepted (which most people are) they will pay the hospital and you will owe them.  Currently, they provide a 90-day same as cash policy but that is subject to change without notice.  Their finance rates vary.  If interested, ask the hospital personnel about the current CareCredit policies and double check on any forms that you sign with them.  The hospital does not offer any direct financing.



The usual period is from 3 to 5 days.



Upon discharge your cat will still be minimally radioactive.  This will decrease to undetectable levels over a period of 2 to 3 weeks.  Even though the level of radioactivity is very low (very much lower than the level at which human patients are discharged from hospitals or offices), you should still exercise prudence during this period.  You should scoop the litter box daily wearing plastic or rubber gloves.  You MUST use flushable litter and dispose of the waste down the toilet.  DO NOT put waste in the regular trash.  There are very sensitive radiation detectors at the dump and the radiation will be detected and an investigation will follow.  If this is not possible, you must collect the litter created during that 2-week period and hold it for 90 days prior to disposal in a dump.  This allows the normal radioactive decay to lower the radioactivity level to none.

You should not sleep with your cat during this period.  Your hands should be washed after petting your cat.  You should limit the time of direct contact with your cat. These recommendations are made pursuant to the “As Low As Reasonably Achievable” policy of radiation exposure as required by the State of Florida.



Yes.  Although it is very uncommon, some cats with extreme hyperthyroidism can have only a partial response to treatment (less than 1% of treated cats) or have reoccurrence.  If there is only a partial response or a reoccurrence, repeat therapy is usually successful.  If repeat therapy is required within the first 2 years, subsequent doses are offered at ½ the original fee.



Almost no cats require specific aftercare.  Your primary care veterinarian will need to test your cat's thyroid in 2 weeks after release from the hospital to your care.  These results usually show a normal to low level of thyroid hormone.  If the level is low, normal levels will likely reappear in 6 to 18 weeks. It is very unlikely that thyroid supplements will ever be needed.  Periodic rechecks should then be sufficient.

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Animal Medical Center at Cooper City