Hyperthyroidism is a condition that causes your cat to overproduce the thyroid hormones. This condition affects many organs in the body, most noticeably the heart.  Thyroid hormones cause the whole body’s metabolism to speed up, the blood flows very quickly, calories burn in excess, weight is lost, hunger and edginess is felt.  The most common signs noted by cat owners are weight loss despite good appetite and restlessness.  Hyperthyroidism must make a kitty feel like we would if we drank high caffeine drinks throughout the day! With hyperthyroidism the gland itself gets large (sometimes we can even feel it) but this change is usually benign. In fact only 1-2% of feline hyperthyroidism is caused by an actual cancerous tumor on the thyroid gland.

With treatment, as kitty’s thyroid hormones become regulated, the heart rate will reduce to a more normal range, the body will burn less calories and your kitty will feel better overall.   It is very important that we regularly monitor the thyroid, liver and kidney levels during our initial initiation of treatment for several reasons.  The first is that not every kitty responds the same way to medication and we need to recheck thyroid levels to manage/find the proper dose and timing (usually twice a day is what is needed).  Secondly, it is very common for liver enzymes to be high when hyperthyroidism is first diagnosed and them to return to normal when the disease is managed. However, if they don’t return to normal then we need to address those enzymes separately. Another reason is that the high heart rate and blood pressure associated with hyperthyroidism increases the kidney filtration rate almost as dialysis would.  Therefore, current kidney disease could be hidden and we need to learn about the actual kidney health when the thyroid is regulated. 

Again, the good news is that once diagnosed, most kitties with hyperthyroidism do very well with treatment and have years of comfortable life ahead of them.  We will work on providing this for your kitty!

Step one will be to start oral medication (Felimazole).  Starting oral medication and then re-evaluating blood and urine levels allows us to know if your kitty is a good candidate for I-131 treatment (which has a single treatment cure rate of 95%) or if we need to stay with medical/nutritional management.

Oral Felimazole (methimazole) manages hyperthyroidism while I-131 has a single treatment cure rate in 95% of cats. Most kitties take Felimazole very well in a treat or canned food.  It can not be crushed or split as humans should not be exposed to the inside of these pills. After starting this medication, 1-3 weeks later a blood test should be submitted to evaluate the thyroid level, complete blood count, kidney and liver levels and a urine specific gravity test performed.  This allows us to adjust medication according to your cat’s response and to determine if the kidneys or liver need any additional support. If we have to adjust the thyroid medication amount or type, then the tests should be repeated again in 1-3 weeks.  Once the levels are stable we run these checks every 6 months. 

A few cats refuse oral Felimazole or have gastroinstestinal side effects (vomiting, diarrhea, inappetance). For these cats, we have the option of transdermal methimazole – a gel rubbed on their ears twice a day to medicate.  This is only prescribed before oral Felimazole if the cat has already demonstrated an absolute unwillingness to take oral medication as oral Felimazole is approved and shown to be the most effective form of methimazole for cats.  However, if gastrointestinal side effects occur, they often go away with transdermal gel.  I-131 treatment is also a great/the best option for these kitties if their kidney and liver function is adequate.  Thyroid surgical removal is most often not recommended as I-131 is a much better option for most cats.  Also, a drug often mentioned on the internet called carbimazole has potentially life-threatening side effects, so it is not recommended as I-131 or surgery are much better and safer options.

For some cats and families that I-131 and methimazole are not an option, there is also a prescription diet called Y/D.  This food manages hyperthyroidism by removing all iodine from the diet as iodine feeds the thyroid.  This food does have success however can not be used for maintenance so other cats who do not have hyperthyroidism can not eat it as their diet.   Also, for it to be effective no other treats (not even pill pockets) or foods can be eaten at all.  There are times when this is a good option for households and there are times when this is not.

Most side effects when starting Felimazole (methimazole) start within the first 2-3 months of treatment so this is the time when we regularly (every 1-3 weeks) perform exams, blood and urine testing.  Most side effects are mild and gastrointestinal are the most common (in-appetite, vomiting).  If in-appetite or vomiting occurs, as discussed above, transdermal gel can be tried and of course I-131, surgery and nutrition are options. If the less common side effects of intense facial itching, lethargy, bleeding (such as nose bleeds) or anemia occurs then the medication must be stopped and either I-131, surgery, or nutritional management will be the only options for treatment.

Please reach out to let us know if you would like to get your cat started on Felimazole and scheduled for follow up visits.  Also, of course if you would like to discuss this on phone/in person or have questions to send back please do. 

I have experienced hyperthyroidism with several of my own cats in my lifetime and want to help you both in this journey.  Here is some more helpful information for you. 

Feline Hyperthyroidism Explained

Hyperthyroidism Treatment Options

More About I-131 Treament

Felimazole Information from the FDA