Hyperthyroidism is a relatively common disorder in older cats with an average age of onset around 12 years.  It occurs when the thyroid glands, which are located in the neck begin to produce too much thyroid hormone.  For most cats this over production of thyroid hormone is caused from a benign enlargement of the thyroid glands.  However, in <2% of cats, this process can be from a cancerous tumor in the thyroid glands.  As a result of this increased thyroid hormone, there is a dramatic increase in metabolic rate resulting in:

  • Weight loss often in spite of increased appetite
  • Increased thirst
  • Increased urination
  • Possible vomiting or diarrhea
  • Unkempt fur or thinning hair coat
  • In end-stages anorexia or lack of appetite

In addition to the clinical signs listed above, the increased thyroid hormone also increases heart rate.  As the heart is stimulated to beat faster and more forcefully, it may enlarge in an attempt to meet these increased demands.  Over time the heart muscle stretches and becomes unhealthy and the increased output of blood may also lead to high blood pressure.  In fact, approximately 25% of cats with hyperthyroidism also have high blood pressure.

What Causes Hyperthyroidism?

Although hyperthyroidism is known to occur more commonly in older cats, there is no known underlying cause.  Some environmental and dietary factors have been heavily investigated and may predispose some cats to developing hyperthyroidism, but the mechanism is unknown.  There are also no specific breeds or sex of cats that more commonly develops hyperthyroidism.

How do you diagnose Hyperthyroidism?

Often the diagnosis of hyperthyroidism is quite straight-forward and involves measuring the blood level of one of the thyroid hormones called the total thyroxine (TT4).  Usually, the TT4 is so high that there is no question as to the diagnosis.  However, there are some cats who have all of the clinical signs of hyperthyroidism but will only have a TT4 in the upper range of normal.  In these cases, a second test, usually either a free T4 by equilibrium dialysis or a T3 suppression test is performed.  If these tests are still inconclusive there are specialized thyroid scans that can be done at a referral center or it may be recommended to recheck a TT4 again in a few weeks to months.

How is Hyperthyroidism Treated?

Because less than 2% of cats with hyperthyroidism have cancer on the thyroid gland, treatment is often quite successful.  There are several treatment options available and the treatment of choice often depends on many factors.  A baseline work-up including blood tests, urinalysis, chest x-rays, ecg and blood pressure as well as echo may be recommended prior to certain treatments depending on the pet’s condition at the time of diagnosis.  Below we have outline some of the treatment options.

  • Oral Medication:  Administering an oral drug called methimazole can help control/regulate hyperthyroidism.  Because methimazole just blocks the production of excess thyroid hormone rather than eliminating the abnormal thyroid tissue, it must be given consistently for the rest of the pet’s life.  While this medication is often quite safe and effective, it can cause side effects in <20% of cats. These may take up to 6 months after starting the medication to manifest. This is part of why frequent rechecks with blood screening are critical within the first few months of starting treatment.  Some of these side effects include:
    • Vomiting
    • Facial itching
    • Lethargy
    • Anorexia
    • Fever
    • Liver damage
    • Anemia and a decrease of white blood cells
  • Radioactive Iodine Therapy: One very effective way to treat hyperthyroidism is with radioactive iodine therapy (I131).  This type of treatment can be done at several referral centers, and involves injecting radioactive iodine into one of the veins.  It targets and destroys abnormal thyroid tissue without endangering other organs.  Because this type of treatment completely eliminates the abnormal thyroid tissue, it often results in complete resolution of hyperthyroidism without the requirement for long-term medication administration.  While failure of treatment or recurrence of disease is possible, this form of treatment has a very high success rate.
  • Surgery: Surgery to remove that abnormal thyroid tissue can also be a very effective form of treatment that has potential to completely resolve the thyroid disease.  This type of advanced procedure would also require referral to a specialty center to be done by a board-certified surgeon.  A thorough baseline work-up should definitely be completed prior to pursuing this type of treatment to ensure the pet is healthy enough to undergo the procedure.  It may also be recommended to place the pet on oral methimazole for several weeks leading up to the procedure to better regulate his or her metabolism before performing surgery.  Surgery can also be quite successful, but does have less than a 5% recurrence rate especially if one lobe of the thyroid gland was normal at the time of surgery, but becomes enlarged 2-4 years later.
  • Hills y/d Diet: Much like with methimazole, this prescription diet is required to be fed for life to regulate thyroid disease.  It limits the intake of critical dietary compounds needed to form thyroid hormone thus reducing the amount of hormone that can be produced.  However, it does have some limitations. This diet must be fed exclusively in order to work effectively.  If the pet is exposed to any other cat foods, treats or table scrabs, this diet most likely will not work.  It also should not be fed as a sole diet to any cats who do not have hyperthyroidism.  Therefore, in a multi-cat household or for pickier cats, this diet may be very challenging to use.

What type of monitoring is involved?

The type of follow-up and monitoring required depends greatly on which type of therapy is chosen. For oral methimazole therapy (the most commonly used treatment), it is recommended to recheck bloodwork at least 1 month, 3 months and 6 months after starting treatment. If dose adjustments are needed, a recheck thyroid level is recommended 1 month after each dose adjustment.  If any other under underlying diseases were noted at the time of diagnosis or in follow-up such as high blood pressure, heart disease or kidney disease, other rechecks may be recommended.

If you feel your pet is exhibiting any signs of hyperthyroidism, or you have more questions regarding this disease or the treatment, the vets and staff at West Park Animal Hospital would be happy to help. As always, the veterinarians at West Park Animal Hospital want you to feel comfortable with the care your pet is getting. We’ve designed all of our appointments to provide ample time to discuss all of your concerns and answer your questions. What is most important to us that we provide your pet with the highest level of care by creating an individual plan for each of patients. If you’d like to discuss more about your pet’s needs, call us at (813) 749-6863 for an appointment or conveniently schedule one on our website.