Hyperthyroidism in Cats

By Dr. Anne Ray, Veterinarian

Thyroid hormones control the rate of many activities in the body. These include how fast you burn calories and how fast your heart beats. All of these activities are your body’s metabolism. If your thyroid is too active, it makes more thyroid hormones than your body needs. This is called hyperthyroidism.

Did you know that cats can suffer from hyperthyroidism too? It mainly occurs in geriatric cats, with most being 13 years or older. Only 3% of affected cats are less than 10 years of age.

There are 2 thyroid glands located in the neck area. One or both can overproduce the hormone. Clinical signs are variable as the thyroid hormone affects most organs. The most common clinical signs are:

  • weight loss
  • increased appetite
  • increased activity or restlessness
  • poor hair coat
  • increased water drinking
  • fast heart rate
  • increased blood pressure
  • weakness
  • occasional vomiting and/or diarrhea
  • heart disease

Diagnosis is made with a blood test to check the thyroid hormone levels. An additional blood panel is done to check other organ function such as kidney, liver and heart. Hyperthyroidism can actually mask pre-existing kidney disease in that the increased blood pressure increases the blood flow to the kidneys. A worsening kidney function can be seen in these cats after the thyroid levels return to normal.

Treatment in these cats is a delicate balancing act. There are 4 types treatment for Hyperthyroidism:

  1. Feeding Hills prescription diet Y/D. This diet has VERY limited iodide. By limiting iodide, there is decreased thyroid hormone produced. This is the ONLY food that can be fed. Regular cat food has enough iodide that just a bite can negate the effects of the prescription food. In multiple cat households this can be a challenge. It comes in canned and dry form. It’s effect can also be temporary in that thyroid levels can start rising again.
  2. Lifelong daily oral or transdermal anti-thyroid medication called methimazole. Most cases require twice a day administration. Discontinuation of medication will result in thyroid hormone levels returning to elevated levels and return of clinical signs. Side effects of methimazole are uncommon but include vomiting, loss of appetite, skin rash and lowering of the white blood cell count. A blood test called Hyperthyroid Monitoring Profile is needed 2-3 weeks after the start of medication as it checks the thyroid hormone level, kidney and liver values as well as the white cell count. It shows if the dose of methimazole needs adjustment. Future monitoring tests are required and the frequency of these tests vary from patient to patient.
  3. Surgical removal of affected thyroid tissue. This requires anesthesia in a geriatric cat that may have other medical problems such as kidney or heart disease. Surgical removal can also result in damage to the adjacent parathyroid gland which controls blood calcium levels. Some cats have thyroid cells in abnormal locations called ectopic thyroid tissue. It can be located in the chest where removal is difficult resulting in recurrence of the hyperthyroid condition.
  4. Radioactive-iodine treatment performed at selected specialty practices, where a day or two of hospitalzation may be required. Radioactve-iodine is given by injection where it accumulates in the abnormal thyroid tissue, sparing the normal tissue. Rarely do thyroid levels become too low requiring oral thyroid supplementation. Cats are not under anesthesia for this treatment.

ALL treatment options require follow-up monitoring blood tests. The prescription diet Y/D may be the first option for early hyperthyroid cats and the lowest starting cost for treatment. As a general rule, when comparing medical management vs. radioactive iodide treatment, methimazole treatment has lower up front cost but more in back end cost (more follow-up visits and blood testing). Radiactive-iodine treatment requires greater up front cost but less back end cost (fewer follow-up visits and blood testing). Because of potential complications of surgery and removal of thyroid tissue, this treatment option can end up costing as much or more than the radioactive-iodide treatment option.

Hyperthyroidism in cats is a treatable disease, but in some cases can be complicated. Our doctors at Rocky Gorge Animal Hospital will discuss treatment options and what might work best for you and your cat. Our goal is to keep you kitty comfortable and happy during his/her golden years.

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