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Hyperthyroidism in Cats

By Jean Duddy, DVM


Do you have a cat that is eating you out of house and home but still losing weight?  Your cat could have hyperthyroidism.

Hyperthyroidism is the most common endocrine disease of the middle aged to older cats.  These cats may appear fine but they lose weight despite having a great appetite.  The disease was first described in 1913 in humans and not until 1979 in cats.  Hyperthyroidism is commonly tested for as part of any geriatric blood tests.

Hyperthyroidism is caused by an over production of thyroid hormone by a growth of the thyroid gland.  Most of these growths (adenomatous hyperplasia) cause the thyroid gland to be larger than normal. The growth also causes excessive production of thyroid hormones.  It is this excess of thyroid hormone that causes the signs and symptoms we see in cats.  Thyroid hormones regulate our metabolism and with excessive amounts the metabolic rate is increased.

  • Besides eating a lot and losing weight, you may notice restlessness, increased vocalization, panting or increased respiration rate, a poor hair coat or gastrointestinal (GI) signs such as vomiting and/or diarrhea. You may also notice increased aggression and changes in sleep pattern.  Any of these abnormalities may trigger a visit to your veterinarian.

On physical exam your veterinarian may notice an enlarged thyroid nodule, weight loss, an increased heart rate or even a new heart murmur (from the heart beating harder and faster).   If it is early in the disease the physical exam may be normal.

Although eating a lot with weight loss is hallmark for hyperthyroidism, a small percentage of cats will show weakness, lethargy and a decreased appetite.  This form of
hyperthyroidism has been called “Apathetic Hyperthyroidism.”

Hyperthyroidism is most commonly diagnosed with a simple blood test called a T4 (thyroxine).  Your veterinarian will also run tests to rule out other common ailments (diabetes, kidney disease, chronic GI disease or cancer), that may have similar signs.  These tests are also part of routine geriatric screening. The tests ordered along with the T4 include:

  • CBC – complete blood count to check red and white blood cells
  • Chemistry –checks for diabetes, liver and kidney values. We often see elevated liver values with hyperthyroidism but we want to see normal kidney values.
  • Urinalysis – to help diagnose diabetes, monitor kidney function and urinary tract infections.

On occasion, the T4 test will be normal but your veterinarian is still suspicious of hyperthyroidism.  There are follow up tests that can be done in these cases.

  • Free T4 (thyroid hormone not bound to protein can be more specific for hyperthyroidism).
  • Thyroid scan (nuclear scintigraphy) – a radioactive scan where technetium-99 binds to hyperfunctioning thyroid tissue. This gives a picture of the thyroid glands and tells how much thyroid tissue is abnormal.

Treatment options for cats with hyperthyroidism include medication, diet, radioactive iodine therapy and surgery.  The best therapy for your cat depends on the severity of the hyperthyroidism, age of the cat and other diseases that your cat may have.  Some additional things to keep in mind are whether the cat will eat the intended diet or if the cat can be medicated. These factors are important in the decision regarding what to try.

Methimazole is a medication that stops thyroid cells from overproducing thyroid hormone.  This medication comes in many forms such as pills, liquid, tasty treats or even transdermal (absorbed through the skin when applied to the inside of the ear).

  • Pro – It is readily available and relatively inexpensive.
  • Con – It does not do anything to get rid of the thyroid growth. Methimazole is a lifelong medication. Some cats will have side effects such as vomiting, diarrhea or inappetance.  Blood tests are done routinely especially for the first several months to be sure the drug is not causing side effects, (low white blood cells, anemia, elevation in liver enzymes) and to be sure that the thyroid level is in the normal range.

Diet – Hills y/d, is very iodine restricted.  The thyroid cells take iodine and make it into thyroid hormone.  With virtually no iodine in the diet, the thyroid cells cannot produce excessive amounts of thyroid hormone.

  • Pro – It is readily available
  • Con – It is the only diet the cat can eat, (no other cat food, no human food, no treats). Diet is lifelong and does not do anything to get rid of the thyroid growth.

Radioactive Iodine (I-131) – is taken up by the hyperfunctioning thyroid tissue.  While these thyroid cells store the radioactive iodine to make thyroid hormone, the radiation kills the thyroid cells.  Because there is a lot of thyroid hormone around, normal thyroid tissue will not be making thyroid hormone so normal thyroid cells will not pick up the radioactive iodine. (See

  • Pro – it is a permanent treatment. No anesthesia required.
  • Con – limited availability in many areas. Larger expense up front to achieve a cure.

Surgery – more than 70% of cats have both right and left thyroids involved in the disease.  Removing both of the thyroid glands at one time puts cats at risk of calcium regulation issues because there are small glands next to thyroid glands called parathyroid glands which may stop working for a while after the parathyroid glands are moved out of the way to get the thyroid out.  Calcium levels can become low for a short period and actually cause tremors and even seizures.  This is why we recommend doing a thyroid scan before any surgery to determine if only one thyroid is involved.  Surgery for removing one gland is very effective.

  • Pro – if only one thyroid gland involved, surgery if very effective. There is minimal stay in the hospital.  A permanent cure is the goal of surgery.
  • Con – anesthesia is involved. Both sides are involved in the majority of cats so a thyroid scan should be done before surgery is done.  There is an upfront cost for the surgery.

Your veterinarian will help you decided which treatment options are best for your cat.  The prognosis for hyperthyroidism is good.


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