Thoughts on Thyroid

Recently my Aunt’s cat, Cosette was diagnosed with Hyperthyroidism. She lives in Washington State and we had talked about her symptoms over the phone; weight loss, drinking more water, and some behavior changes. She was evaluated by her veterinarian in Sequim and the blood tests confirmed that Cosette’s Thyroid level was too high.

She has started treatment and is doing well now – so I thought it would be a good idea to have a review of this common condition we see in many middle aged or older cats at our clinic. Our supper blogger Courtney Williams put this together.

Feline Hyperthyroidism

Hyperthyroidism is one of the most common diseases we see in cats, along with diabetes and kidney disease. Your cat’s thyroid glands are a pair of butterfly-shaped glands at the base of the neck, where the trachea (windpipe) enters the chest. A normal thyroid gland secretes just the right amount of hormone, called Thyroxine (T4), which keeps the body working like a well-oiled machine, regulating the body’s metabolism and organ function. Problems arise in cats due the growth of a benign tumor located on the right or left thyroid lobe. Luckily, only about 3-5% of these tumors are malignant. The tumor causes the thyroid to produce far too much Thyroxine, which circulates throughout the body, leading to a myriad of imbalances. Generally, it is a disease of older cats with the average age being 13 years old. There is no breed or gender predisposition, however some studies say Siamese and Himalayan cats have a decreased incidence.

The following are a few signs you might see at home:

  • Ravenous appetite
  • Increased thirst
  • Increased or inappropriate urination
  • Restlessness or hyperactivity
  • Weight loss
  • Chronic, intermittent vomiting or diarrhea
  • Prominent thyroid gland (sometimes can be felt during physical exam)
  • Yowling or increased vocalization

In order to diagnose your cat with Hyperthyroidism, we will need to gather an extensive patient history, perform a physical exam, and run a few diagnostic tests. The lab tests we run in- house will evaluate metabolic function, hematology, and a thyroid level. If the thyroid level is markedly elevated, then the diagnosis is clear. However, occasionally we will see borderline levels, which will require further testing. Additional testing of the thyroid, kidneys, or heart function might be recommended by the doctor depending on several factors. If your cat’s test results confirm the diagnosis of hyperthyroidism, there is plenty that can be done about it. The 3 treatment options available are listed below. It is very important to consider every option, as they all come with their own set of advantages and disadvantages.

Oral Methimazole (Tapazole or Felimazole)

Methimazole is a medication that blocks the production of T4 and T3 hormones, and helps the body return to normal levels. It is administered 1-2 times daily by mouth, and patients will need to take it life-long. Anyone who has owned a cat knows that giving them a pill twice daily is much easier said than done. If your cat refuses to take pills, the Methimazole can be compounded into a liquid or transdermal form through the compounding pharmacy.


  • Oral Methimazole is relatively inexpensive
  • Side effects are very uncommon
  • 1 or 2 doses can be skipped with little consequence
  • Can be discontinued if needed


  • Can be difficult to administer
  • Compounding the medication (if needed) is typically more expensive
  • Possible, yet rare, side effects include: lethargy, loss of appetite, vomiting, or facial itching.
  • Less than 2-4% chance of liver failure or bone marrow changes
  • Requires periodic blood testing (every 6-12 months)
  • Radioactive Iodine Therapy(I-131)

This therapy is the most sophisticated and state-of-the-art treatment option available. Your cat is given an injection under the skin (much like a vaccine) of Radioactive Iodine (I-131). This medication becomes concentrated within the thyroid gland, where it irradiates and destroys the hyper-functioning tissue. The patient has to stay in the hospital for an average of 5-10 days, or until the level of radioactivity in their urine and stool decreases to an acceptable level. A successful treatment is curative.


  • Avoids the inconvenience of daily, oral medications
  • Stress free for most cats
  • Less potential side effects
  • Safer and easier than surgery
  • In over 90% of cases, only a single administration is curative


  • Hospitalization is required, no visiting allowed due to radiation
  • If the patient is UNDER dosed, they will persistently remain Hyperthyroid. On the other hand, if a patient OVER dosed, they can become Hypothyroid (not enough hormone), and would be required to take supplemental Thyroxine daily.
  • Expense. Treatment is roughly $1000. However, may be less of an investment than oral medications over the cat’s lifetime
  • The patient must be completely stabilized in order to undergo treatment.
  • Not all facilities offer this treatment. Special licensing and regulation is required.
  • Surgical Treatment

Surgery can be performed to remove abnormal thyroid tissue, and leave normal adjacent tissue. A test called a Nuclear Medicine Scan can be done to determine exactly which thyroid lobe should be removed. This decision can also be made by the surgeon during surgery based on visual appearance.


  • Surgery is generally permanent, unless unforeseen complications arise
  • There are more facilities available to perform the procedure, unlike I-131 therapy.


  • Anesthetic risk. As discussed in the link above, cats with heart disease, kidney disease, or high blood pressure will have a greater risk for anesthetic complications
  • If any abnormal tissue is left behind, Hyperthyroidism is likely to reoccur within 6-24 months. If too much tissue is taken, the cat is likely to become Hypothyroid (doesn’t produce enough thyroid)
  • Requires continued T4 monitoring twice yearly.
  • Potential damage to adjacent nerves
  • Calcium Crisis in cats that require removal of both lobes.

Prescription Diet

A new prescription diet was introduced a few years ago called Y/D that is made with almost no iodine. The idea is that if a hyperthyroid cat was fed this diet EXCLUSIVELY, the hyperactive thyroid gland would stop overproducing T4 hormone and the symptoms would be controlled. This diet has worked well for some cats but palatability issues and concern for iodine deficiency in other organ systems has limited the use of this treatment option. Finding the right form of treatment for your cat will be vital to their overall health for years to come. It is important to understand that this disease will progress quickly if not treated. Cats with untreated hyperthyroidism have a greatly reduced quality of life. They will experience weight loss, muscle deterioration, chronic vomiting/diarrhea, heart disease or high blood pressure. All these things can result in heart failure, sudden blindness, and sudden death. The good news here is that Hyperthyroidism is by NO means a death sentence for your cat. With prompt and consistent treatment they will go on to live a happy and healthy life for many years to come!

What's Next

  • 1

    Call us or schedule an appointment online.

  • 2

    Meet with a doctor for an initial exam.

  • 3

    Put a plan together for your pet.