Hyperthyroid cat getting radioactive Iodine injectionInjection of radioactive iodine, I-131An Explosion of Thyroid Disease Since the 1970’s

Before the 1970’s hyperthyroidism was not a common condition seen in our feline pets. So what’s changed to cause such a dramatic increase in diagnosis? The answer is that we just don’t know. Some suspect that it may be related to the chemicals that we surround ourselves with every day such as the plastics that line the cat food cans or the flame retardant chemicals in our clothes, rugs, and mattresses. As researchers continue to study the possible causes of this endocrine disease, veterinarians and pet owners need to focus on how to diagnose and manage hyperthyroidism successfully.

What is Hyperthyroid Disease?

Hyperthyroidism is a common disease diagnosed in many of our older feline patients (average age at diagnosis is 13 years old). In these patients an excessive amount of the hormone thyroxine, also known as T4, is produced by overactive thyroid glands leading to a variety of symptoms which include:

  • weight loss
  • increased appetite
  • increased activity or restless behavior
  • “crabby” or aggressive behavior
  • increased heart rate
  • GI signs such as vomiting and/or diarrhea
  • increased urination

This excess of the hormone that regulates an animal’s metabolic rate is a result of abnormal growth of thyroid cells referred to as a thyroid adenoma. In most cases theses growths are benign and very responsive to treatment. In 1-2% of cases the abnormal growths are malignant and may involve different treatment options than will be discussed today.

How is Hyperthyroid Disease Diagnosed?

A complete workup including a CBC, Chemistry, Urinalysis, and T4 level are necessary to evaluate a pet with suspected thyroid disease. Sometimes other illnesses such as diabetes, heart disease, and liver disease can produce similar symptoms as seen in thyroid disease. The thyroid level itself can be affected by other concurrent disease such as IBS or some forms of cancer. Evaluating the entire pet is essential to guiding proper treatment and addressing all potential health concerns.

What are the Treatment Options?

There are 3 basic treatments for the hyperthyroid cat.

1. Supplementation with medication called Methimazole.

This medication (also known as Tapazole) can be given as a pill, liquid, or transdermal cream on the inside ear. Treating with medication is for the life of the pet and will require periodic monitoring bloodwork on a regular basis. Medicating any cat can be a challenge, but every day for the rest of their life may not fit every owner. While very effective in managing hyperthyroid disease, 10-20% of patients on methimazole experience drug reactions such as vomiting, skin irritations, and blood changes.

2. Surgical removal of enlarged thyroid gland.

This option is not without its own set of complications. It is rare that surgery completely results in a normal thyroid level. Small clusters of thyroid tissue can be anywhere in the body and continue to produce an elevated T4. On the other hand, if both thyroid glands in the neck are removed there is also a risk of the cat developing hypothyroid disease with too little T4 circulating. Hypothyroid disease requires daily supplementation and we are back to medicating the difficult cat.

3. Radioactive Iodine Therapy.

This is the treatment of choice for many veterinary professionals if available in your area (which there is, of course!). A single injection of radioactive iodine (isotope I-131) is injected under the skin of a patient and then concentrates in the overactive tissue of the thyroid gland. Healthy tissue of the thyroid and parathyroid glands are left alone. The overactive thyroid tissue is irradiated and destroyed leaving only healthy cells behind. In most cases this is a one-time treatment and a normal T4 level is reached about 1 month after treatment. Pets treated with I-131 will be hospitalized for 4-7 days initially due to radiation levels. At discharge owners will be sent with special handling instructions of the patient and their waste products to be followed over the next 2 weeks. This mostly entails purchasing a box of latex gloves from the pharmacy and setting up a dedicated trash bag to collect litter box waste for 2 weeks. The price of treatment is a limiting factor for some pet owners initially until you add up the other options. The cost of daily medication for potentially years combined with the cost of regular monitoring bloodwork and recheck visits make the radioactive treatment option at $925 seem a little more reasonable. It is not common, but in rare cases cats with very high T4 levels may need a second injection of I-131 to treat any residual abnormal tissue.

Choices are Good

Unfortunately, hyperthyroid disease looks like it is here to stay. It is great that we have some very effective treatment options to choose from for our feline friends. Every option has its pros and cons to consider. I am sure that after teaming up with your pet’s veterinary professional, you will find the solution that will fit the needs of your furry family if hyperthyroidism is diagnosed in your pet.