Feline Hyperthyroidism

What is Feline Hyperthyroidism?

The thyroid is a butterfly shaped gland located adjacent to the trachea in the throat area. These glands produce hormones (T3 & T4) that control the metabolic rate. When a cat becomes hyperthyroid, these glands produce an excess of these hormones, which accelerates wear and tear on the cat’s internal organs. In most cases, hyperthyroidism is caused by a benign tumor of the thyroid gland.
What are the Signs of Feline Hyperthyroidism?

Clinical signs in your cat are the result of “run-away” metabolism and include:
  • Cardiomyopathy (heart disease)
  • Hypertension (high blood pressure)
  • Gastrointestinal Disorders (vomiting, diarrhea)
  • Dermatopathy (skin disorders, hair loss)
  • Osteoporosis (loss of bone density)
  • Renal Disease (kidney disease)
  • Behavior Changes (hyperactivity, aggression, depression or apathy)
  • Inappropriate Elimination
  • Excessive Thirst and Urination
How Is Hyperthyroidism Diagnosed?

A thorough physical examination once yearly (or semi-annually for cats over 10 years of age) may alert your vet to subtle changes in your cat. Annual screening with blood work including thyroid hormone (T4) testing, blood pressure measurement and palpation of the thyroid glands is essential.  Often hyperthyroidism is detected on routine geriatric screening or pre-dental blood work.  Diagnosing the disease earlier rather than later is ideal and will help decrease negative, irreverable effects on your cat’s health.
Radioactive Iodine Therapy

Radioactive iodine therapy is now considered the treatment of choice for feline hyperthyroidism. The therapy is 96% effective with one treatment. Rarely a cat may need a second dose. 
How Do We Identify a Good Candidate for Radioactive Iodine Therapy?

Age is not a factor. The cat must be in reasonably good health, that is, have no underlying disease that will be adversely affected when the metabolism and blood pressure returns to normal. Prior to receiving I-131 patients will have a complete work-up including physical exam, blood and urine testing, and blood pressure reading.  If your pet has a heart murmur or known heart condition, chest radiographs and echocardiogram are recommended.  The goal is to understand the extent of their problems related to hyperthyroidism and rule out any other concurrent disease processes like diabetes, renal disease, cardiac disease, etc. prior to radioactive iodine therapy.  We will work with your veterinarian to complete the screening process and manage any concurrent diseases if necessary.
How Does the Therapy Work?

The dose of 131-iodine is administered subcutaneously between the shoulders. The 131 iodine is absorbed immediately to the circulation where it saturates the abnormal thyroid cells for about 12 hours. Normal thyroid cells do not absorb the iodine because they are atrophied. Since the thyroid is the only tissue that utilizes iodine, only the active (abnormal) thyroid cells will absorb the iodine. Over the next 36 �" 48 hours, the iodine that has not been absorbed by the thyroid cells will be excreted by the cat primarily into the urine. This is why they must remain in our care for this period. Within days the abnormal cells will be destroyed by the intracellular beta radiation and the T4 levels will start to decline.
Can I Visit My Cat?

We admit our patients Monday morning between 10:30am and 12:30pm and administer the dose between 1pm and 2pm.   Up to 4 cats will be housed in a boarding wing that has been specially constructed to shield radiation.  Our patients will stay with us for 4 days to allows the radioactive material time to decay to a safe level and can go home Thursday afternoon or evening or be moved into our extend stay boarding room.  Only certain team members that are certified by the State of Oregon can care for these patients during the 1st 4 days.  We limit physical contact with our I-131 patients to about 15 minutes twice a day, enough time to assess each patient, provide clean & comfortable housing, food, water, and social interaction.  We have cameras for out of the room observation and monitoring.  You may leave small toys and blankets for your cat but, we will be unable to return them to you.  The I-131 coordinator will provide you with a daily update, either by phone or email.  If your cat stays with us for extended care boarding, you may visit once a day for 15 minutes.
What Side Effects Might I See?

While there are no direct side effects, we sometimes see the cat become quiet, almost lethargic and eat less during a transient period before the atrophied normal tissue begins to make hormone. We rarely see a cat develop a “sore throat” 3-7 days after the therapy. This is the result of the mild inflammatory reaction in the thyroid while the body is absorbing the dying thyroid cells. We call this radiation thyroiditis. It is usually self-limiting with anti-inflammatory medication. With thyroiditis, the cat appears hungry but may have difficulty swallowing. Finally, rarely, we see a change in vocalization (hoarse meow) that is usually transient.
What Home Care Will My Cat Need After Therapy?

Although your cat will be discharged at radiation levels deemed safe by state and federal guidelines, a few contact, handling and litter protocols will need to be followed for 1 week after discharge. If for any reason you are not able to adhere to these guidelines, our extended stay boarding may be an option for your household.  We highly encourage extended stay boarding if you have a cat that has a history of inappropriate urination.  Please refer to the home care and litter box hand outs below.
What Follow Up Does My Cat Need?

Each cat is unique!  Your cat’s follow up plan will be based upon findings from their initial screening.  Typically thyroid levels need to be measured at 30 and 90 days, then again at 6 months post-therapy.  Occasionally, Dr. Yuko may request that your cat be seen sooner than 30 days if they have other concurrent medical issues. We will provide you and your veterinarian a written summary at time of discharge that will include your pet’s ideal monitoring plan.  We encourage you to do the follow up care with your regular veterinarian and Dr. Yuko will be available to consult as needed.