Feline Hyperthyroidism 101 With Dr. Lorie Huston

Gris Gris-hyperthyoidism-cats-feline

Gris Gris diagnosed with hyperthyroidism

This week, we’re delighted to have veterinarian Dr. Lorie Huston guest post for us again. Vet 101 Q & A will return next week. Vets are seeing an increase in feline hyperthyroidism in recent years. Find out what you need to know about this treatable disease.

Hyperthyroidism in Cats

Hyperthyroidism is a common disease in cats, particularly in older cats. It is caused by an over-active thyroid gland that is secreting an abnormally high level of thyroid hormone. It is the effect of these elevated thyroid hormones on the rest of the cat’s body that causes the symptoms seen.


The symptoms seen may be variable but include:

  • Weight loss despite a good or even voracious appetite
  • Vomiting
  • Diarrhea
  • Muscle wasting
  • Heart disease/heart failure
  • High blood pressure (hypertension)
  • Blindness (resulting from hypertension)


The exact cause of hyperthyroidism is still unknown. Here’s what we know.

  • Cats with hyperthyroidism usually have a benign tumor in the thyroid gland that causes the excess secretion of thyroid hormone.
  • Feeding canned food from “pop-top” cans has been correlated with an increased incidence of hyperthyroidism but not all cats that develop hyperthyroidism have been fed these types of foods.
  • There has been some speculation that flame retardants (polybromated diphenyl ethers, PBDE) may have contributed to the increased incidence of hyperthyroidism. However, blood testing has failed to detect differing levels of the chemical in the blood of cats with and without hyperthyroidism.
  • Additional theories speculate that other household chemicals could be responsible.
  • Some people theorize that cats are simply liver longer today than previously. Because hyperthyroidism is primarily a disease of older cats, it’s possible that more cats are simply living long enough to develop the disease now.
  • There is also some train of thought that hyperthyroidism is now a more commonly known disease than it was previously and that veterinarians are testing for it more often, leading to an increased number of diagnosed cats.


The definitive test for hyperthyroidism is a blood test known as total T4. If the total T4 level is increased, the cat is hyperthyroid. However, sometimes blood levels are borderline and additional testing may be required. This additional testing may be a blood test known as free T4.

For most cats, a complete blood screen (which includes a complete blood cell count, blood chemistry profile and electrolyte levels) will be performed simultaneous with the total T4 measurement. This blood screen examines red and white blood cell values, liver enzymes, kidney function tests and electrolytes such as sodium, potassium, phosphorus and calcium.

If heart disease is suspected, radiographs (x-rays) of the chest and/or an echocardiogram (an ultrasound study of the heart) may be recommended.


There are several treatment options available for feline hyperthyroidism.

  • Methimazole (Tapazole, Felimazole) is a drug that is commonly used to treat hyperthyroidism. For most cats, this medication is relatively effective in controlling the signs of disease. However, the medication is not curative and cats treated with methimazole need to receive the drug for their entire lifetime.
  • Radio-iodine therapy is a technique which uses a radioactive drug to destroy the unhealthy cells in the thyroid gland. This treatment is curative but it is not inexpensive and does require hospitalization in a specialized facility qualified to perform this type of treatment.
  • Surgical removal of the thyroid gland is also possible and is still offered as a treatment option in some cases. However, radio-iodine treatment has largely replaced surgery as the treatment of choice when cure of the disease is desired.
  • Hyperthyroidism can also be treated through the use of a specialized diet (Feline Y/D) that restricts the cat’s dietary iodine intake. It is important that only cats suffering from hyperthyroidism consume this diet. In healthy cats, this diet is not nutritionally complete and may lead to serious disease.

In some cases, hyperthyroid cats are started on methimazole as a preliminary treatment to observe their response to lowered thyroid hormone levels in the blood. Kidney disease can sometimes be masked by hyperthyroidism and symptoms of kidney disease sometimes become evident after treatment is instituted. If this is the case, methimazole can be discontinued temporarily or the dosage lowered to allow recovery.

Cats that do well on methimazole may go on to receive radio-iodine therapy if their owner elects a curative option. Cats treated with radio-iodine therapy will not require further administration of methimazole.

About Lorie Huston: Lorie is a veterinarian with over 20 years experience with dogs and cats. She currently practices at a busy urban practice in Providence, RI. Besides being a veterinarian, Lorie is a talented writer and blogger. You can learn more from Lorie about pet care at her blog, Pet Health Care Gazette, or find her at Lorie Huston.

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