Hyperthyroidism in Cats
Dr. Lisa King, DVM
Animal Hospital of Towne Lake 


Your cat has been diagnosed with Hyperthyroidism. To start treatment we have prescribed Methimazole, also known as Tapazole. After starting medication, as long as your pet is eating, drinking and tolerating medication well you and your cat need to return in 3 weeks for a weight check, update and for us to collect blood to send to the laboratory to recheck a CBC, kidney enzymes, urinalysis and T4 (thyroid) level. If you are not able to give the medication, please call to alert the medical staff for advice on techniques of administration or alternate forms of medication. The most common, cost effective treatment owners choose to treat their pet is to administer the medication as a tablet, usually twice daily. Alternate forms of medication available are to have the Methimazole compounded into a liquid, chewable or transdermal lotion type medication rubbed onto the ear.

If you have not been able to consistently give the medication as directed you will need to alert the medical staff as to when to recheck your lab work. The bloodwork needs to be rechecked soon after starting or if ever changing doses of the medication. Fine tuning and dose adjustments of the medication as needed are based on the blood levels. Giving too much or too little medication will not achieve the goal of control. Once the thyroid is controlled with medication, the patient and blood work are checked every 6-12 months or more often if dose changes are made.


You should stop Methimazole treatment and call the clinic if you see any of the following:


               Prolonged Vomiting                          Bruises on gums or belly

               Lack of appetite                                   Pale or Listless 

               Open Mouth Breathing                     Increased water intake


The long term therapy options for Hyperthyroidism treatment are continued daily Methimazole, Radioactive Iodine given as an injection, surgery or a specialized prescription Thyroid food called Y/D. Your Veterinarian will discuss these options with you to pick which is right for your cat. Please read the following to better understand Hyperthyroidism.

The thyroid is a two-lobed gland located in the neck of humans and animals, one lobe on each side of the wind pipe. The thyroid produces a hormone that is transported via the blood to every cell in the body. The primary function of the thyroid hormone is to control the rate at which cells function. Too much makes the cells work fast. Too little makes the cell function slow down. Low thyroid function (Hypothyroidism) is common in dogs and quite rare in cats. Excess thyroid function (Hyperthyroidism) is one of the most common diseases diagnosed in cats 8 years of age and older.

Common owner observations of cats with Hyperthyroidism are weight loss, increased appetite and water intake, high blood pressure, HCM (Hypertrophic Cardiomyopathy), patchy hair loss, failure to groom, increased urination and increased activity. Less Common problems are vomiting and diarrhea, panting, loss of appetite, listlessness, changes in behavior and seeking cool places. It is not known exactly why cats develop Hyperthyroidism. About 15% of cats have a single non-cancerous tumor, and 80% have excessive activity in both lobes, a benign condition. 3-5% have a malignant (cancerous) thyroid tumor. Diagnosing hyperthyroidism in cats is relatively easy. Most hyperthyroid cats have too much thyroid hormone (T4) in their blood, and this can be confirmed with a blood test.


Hyperthyroid cats that are not treated tend to become more and more ill, but treatment has shown that your cat can return to a reasonable state of good health or be cured.


4 Currently Available Treatment Options:


Methimazole is a prescription only medication that works by preventing the thyroid gland from creating T4. This drug is easily available and not very expensive. Since some cats are not the best pill takers, other forms of Methimazole are available such as compounding into liquid, transdermal gel or flavored chewable. Uncommon side effects of Methimazole are liver damage, decreased red and white blood cells and platelets (cells that help the blood to clot) and self-induced trauma. Although these side effects are alarming, they are not common and we still use this medication for almost every hyperthyroid cat we treat.


Radioactive Iodine Treatment is an extremely effective treatment that resolves the Hyperthyroidism quickly in 96-98% of cats cured and requires no anesthesia or pills. However 5% will become Hypothyroid and require oral thyroid supplementation. Iodine is the primary ingredient of the thyroid hormone. The negative aspect of treatment are that sophisticated facilities are needed (not performed with regular DVM) and the pets usually require 3-7 days of hospitalization at the facility to eliminate the radioactivity in the urine and feces as special handling of the feces and urine are necessary (can be harmful to humans).


Surgery to remove the abnormal thyroid gland is the third option. The surgery is not difficult and the hyperthyroidism resolves quickly and can cure your cat permanently. the negative aspect of surgical removal is that anesthesia is required and many hyperthyroid cats are older with other health problems that could cause complications. Other risks are the accidental removal of the parathyroid glands (necessary for life) and, as with any treatment, the surgery may not be successful.


Hill’s Y/D Diet is a prescription diet which has limited iodine content which is the main ingredient in the thyroid hormone. It is also designed to support heart and kidney function. This method treats/supplements with food with no pills, surgery or radioactive iodine medication. Unfortunately, there may be problems with feeding in a multiple cat household. A hyperthyroid cat can eat only the Y/D food and it is NOT recommended to feed to cats that do not have thyroid problems.

Owners sometimes report that their cat will not eat the food. Cats are resistant to change and it could take weeks up to months to change completely over to the Y/D diet. For long term control, no other foods or flavor enhancers can be can be added to the food. We can give helpful tips to transition to the new food. The food comes with a 100% money back guarantee from Science Diet if you cat will not eat the food. You would be refunded the full cost of the food. Methimazole oral supplementation CAN be used in conjunction with the Y/D diet but careful adjustments of the medication are required. You will need to ask the Doctor for dose adjustment recommendations if you decide to feed the diet AND do oral supplementation. You would not be giving the same dose of medication if your cat is on the food.


***Specific requirements to make the Y/D diet approach as successful as possible:

No other treats or food at all

Patient must eat of out a stainless steel bowl

NEW food storage container – cannot store in container that regular food has previously been stored in

NEW food scooper – cannot use old scooper from previous food


It is important to understand that all the treatments are excellent and all also have their negative aspects. It is also important to know that the blood supply to the kidneys will decrease after successful treatment of Hyperthyroidism. This is why it is very important we monitor both the thyroid level and kidney function via blood work soon after starting treatment. We begin treatment with the Methimazole pills/oral medication before surgery or radioactive iodine treatment because the effects of the oral medication are reversible. If decreasing the thyroid hormone levels reveals kidney damage, a permanent form of treatment (radioactive treatment or surgery)is avoided. If the thyroid hormone levels and the kidney function is unaffected, a permanent treatment plan is an option.