Dechra Pharmaceuticals

What is Hyperthyroidism?

Hyperthyroidism is one of the most commonly diagnosed endocrine conditions, particularly in older cats. Cats with this disease produce an excessive amount of thyroid hormones – thyroxine (T4) and triiodothyronine (T3) – which adversely affects virtually every organ system and causes a dramatic increase in the body’s metabolic rate. Thyroid hormones are produced in the thyroid gland, which is made up of two lobes located on either side of the windpipe (trachea). Hyperthyroidism occurs when either one or both of the lobes become enlarged. This enlargement leads to the overproduction of thyroid hormones. The reasons for the change in the thyroid gland are not fully known. In 98% of cases, the enlargement of the lobe(s) is benign. In these cases, diagnosis and treatment are generally straightforward and successful. However, in the remaining 2% of cases the enlargement is due to a malignant (cancerous) growth of the thyroid gland. Regardless of the cause of the enlargement, the result is the same: excessive amounts of thyroid hormones are produced that can lead to serious medical problems for your cat.

Diagnosing the Disease

When hyperthyroidism is suspected, diagnosing the disease generally is not complicated. In many cases, your veterinarian will be able to feel the enlarged thyroid lobes along your cat’s neck. Clinical signs usually associated with hyperthyroidism may include:

  • Weight loss
  • Increased appetite without weight gain
  • Diarrhea
  • Vomiting
  • Increased thirst
  • Frequent urination
  • Increased restlessness and irritability
  • An unkempt hair coat
  • Anxious facial expression
  • Rapid heart rate

Since not all cats will react to the disease in the same way, your cat may or may not demonstrate all of the symptoms. To confirm the diagnosis, your veterinarian will measure the level of thyroid hormones and perform routine blood tests to assess the function of other organs. The presence of disease in other organs may influence the response to treatment for hyperthyroidism.

The Importance of Treatment

Hyperthyroidism affects the function of virtually every organ system in your cat’s body. If untreated, your cat may develop serious conditions such as:

  • Heart disease
  • Emaciation
  • High blood pressure
  • Muscle wasting
  • Severe metabolic dysfunction

Managing Hyperthyroidism

Many veterinary endocrinologists recommend that cats with hyperthyroidism receive methimazole for a trial period of at least 3 weeks. This allows them to be stabilized before making a decision about long-term therapy, which may include surgery or radioactive iodine treatment. Hyperthyroidism can also be successfully treated long term with medication. Felimazole Coated Tablets are the first FDA-approved treatment for managing hyperthyroidism.

With Felimazole Coated Tablets, you and your veterinarian can return metabolism and thyroid levels to normal and help your cat experience an improved quality of life for years to come.

The First Steps to Good Health

The sooner your cat begins treatment, the sooner your cat will get relief from the clinical signs of hyperthyroidism.

You will begin treating your cat’s hyperthyroidism with Felimazole Coated Tablets.

In 3 weeks:
Your veterinarian will assess your cat’s response to Felimazole Coated Tablets by evaluating the clinical signs and performing blood tests. Your veterinarian may adjust the dose at this time.

In 6 weeks:
You will schedule an appointment with your veterinarian to continue monitoring the early stages of therapy and, if needed, make adjustments in the dosage of Felimazole Coated Tablets. Felimazole Coated Tablets provide the dosing flexibility to meet your cat’s specific needs.

In most cases, within 3 to 6 weeks of starting therapy with Felimazole Coated Tablets, you can expect to see the following improvements:

  • Decrease in excessive appetite
  • Decrease in hyperactivity
  • Decrease in anxiousness
  • Decrease in water consumption
  • Decrease in urination
  • Decrease in vomiting and diarrhea
  • Improved hair coat

Once you and your veterinarian are happy with your cat’s progress, your veterinarian will establish an ongoing monitoring schedule. However, you should call your veterinarian if the clinical signs of hyperthyroidism recur or if yourcat becomes ill.

Frequently Asked Questions

Q: What is Felimazole and how does it work?
A: Felimazole is the first FDA veterinary approved treatment for feline hyperthyroidism. It contains the anti-thyroid drug methimazole, which reduces the production of the hormones T4 and T3 within the thyroid gland.

Q: What should I do if I forget to give a tablet?
A: Do not give a double dose the next day. Give the prescribed dose of Felimazole Coated Tablets at the regular dosing time.

Q: How long will my cat require treatment?
A: Most cats need to be given Felimazole Coated Tablets for life.

Q: Will I need to revisit my veterinarian?
A: Yes. It is important that your veterinarian monitors your cat and the dose of FELIMAZOLE Coated Tablets. Evaluations should be done at 3 weeks and 6 weeks after starting treatment. Once your cat is stable, your veterinarian will recommend periodic evaluations.

Q: Does Felimazole have side effects?
A: Side effects most commonly occur within the first 3 months of therapy. They are generally mild and may include decreased appetite, vomiting, and lethargy. More serious side effects are possible, so if your cat becomes ill at any time while being treated with Felimazole Coated Tablets, stop treatment and consult your veterinarian as soon as possible.

Treatment Tips


  • Take your cat back to your veterinarian for regular monitoring.
  • Give your cat the prescribed dose of Felimazole Coated Tablets every day.
  • Contact your veterinarian immediately if your cat stops eating or becomes ill while receiving Felimazole Coated Tablets.
  • Wash hands with soap and water after administering Felimazole Coated Tablets or after handling litter used by treated cats.
  • Wear gloves when handling litter of treated cats if you are a woman of childbearing age.


  • Change the dose without first consulting your veterinarian.
  • Crush tablets.
  • Stop administering Felimazole Coated Tablets, even if you notice dramatic physical improvements in your cat’s health.

Felimazole Coated Tablets resolve the symptoms of hyperthyroidism, but they do not cure the underlying disease.

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