Feline Hyperthyroidism

What is Hyperthyroidism?

Hyperthyroidism is the most common endocrine disorder in cats, and is one of the most common diseases of older cats. Most cats diagnosed with hyperthyroidism are over 10 years old. Our cat Barney was treated for both hyperthyroidism and diabetes, and it's not unusual for a cat to have both diseases.

Hyperthyroidism occurs when the thyroid gland, a small double-lobed gland in the neck, produces an excess amount of thyroid hormones.  The thyroid gland produces two hormones, thyroxine (T4) and triiodithyronine (T3).  In most cats, hyperthyroidism is the result of too many active cells in the thyroid gland.  Very rarely is it caused by thyroid cancer (1-2%).  Treatment of hyperthyroidism is usually very successful.


Typical signs of hyperthyroidism include

  • weight loss
  • increased appetite
  • vomiting
  • excessive drinking
  • excessive urination
  • hyperactivity
  • diarrhea
  • the coat may look dirty and dull
  • some cats have very rapid toenail growth or thick nails
  • some cats display aggression
  • a few cats (less than 10 percent) have decreased appetite, depression, and weakness

In addition to the signs listed above, hyperthyroidism causes

  • increased blood pressure
  • increased stress on the heart: rapid heartbeat and strong heart contractions, or irregular heartbeat

Untreated hyperthyroidism can lead to

  • congestive heart failure
  • kidney damage
  • diarrhea that is very difficult to treat
  • detached retina (due to high blood pressure) which could lead to sudden blindness
  • death


The levels of thyroid hormone in the body are measured by a blood test. Other blood tests and methods of imaging the thyroid gland may be performed.

Treatment options

There are three treatment options available, and the method used will will depend on the pet.


Medication prevents the thyroid gland from producing excess thyroid hormones. Methimazole (Tapazole) is a commonly used antithyroid drug. Normal thyroid hormone levels are usually achieved in 1-3 weeks.  Side effects include anorexia and vomiting, but these often lessen with time. It may be possible to decrease the dose to lessen the side effects while still maintaining proper thyroid hormone levels. Additionally, some cats may become hyperthyroid again, even while taking medication. Hospitalization is usually not required.


Surgery involves partial or total removal of the thyroid gland. Normal thyroid hormone levels are usually achieved in 1-2 days.  Side effects include surgical risks, and hypothyroidism (low thyroid hormone). The parathyroid gland (a different gland next to the thyroid gland) may also be removed or damaged, resulting in hypoparathyroidism. Because hyperthyroid cats may have increased stress on their heart, they are usually started on antithyroid medication prior to surgery so the thyroid hormone levels are normal and the effects on the heart are minimized. The cat is hospitalized for 1-3 days.

Radioactive iodine treatment

Radioactive iodine is used to kill the overactive thyroid cells. Normal thyroid hormone levels are usually achieved in 1-12 weeks. This is considered the best treatment if a facility is available. It is very low risk, but not all cats are candidates, particularly cats with other health problems that require them to be handled frequently.  After radioactive iodine treatment, the cats can't be handled right away because they must eliminate the radioactive iodine first.  This makes it difficult for a diabetic cat, who requires insulin injections, to be a candidate for radioactive iodine treatment. Also, few facilities are available for this treatment, and the cat must be hospitalized for 1-4 weeks.

Considerations for Diabetics

Since both hyperthyroidism and diabetes are common in older cats it is often necessary to deal with both conditions.  Usually, the cat has one disease, which is being treated, then the second disease begins and must be dealt with.

If your cat is hyperthyroid first, you will probably have started therapy and gotten the thyroid hormone levels near normal. Then if the cat then becomes diabetic, the hyperthyroidism (which is controlled) probably won't play a major role in the diabetes management.

The situation is a bit more complicated if the cat is diabetic, then becomes hyperthyroid too.  Since hyperthyroidism has similar signs as diabetes (increased appetite, drinking, urination, and weight loss), a cat that is diabetic who continues to show these signs may be thought of as just needing additional diabetes management care.  But the possibility of hyperthyroidism should not be overlooked.  Since hyperthyroidism causes a general increase in the body's metabolic rate, it will have an effect on the diabetes and diabetes management. Often, a cat that is hyperthyroid requires higher doses of insulin in order to maintain the blood glucose levels near normal.  As the hyperthyroidism is brought under control, the metabolism slows to a more normal rate, and the insulin dose needs to be adjusted.  Pre-existing diabetes will probably have an effect on which hyperthyroid treatment option is used.

Barney - diabetic, hyperthyroidism treated with Tapazole

Barney was diagnosed with hyperthyroidism 6 months after being diagnosed diabetic. The hyperthyroidism was detected during a routine pre-anesthesia blood test (he needed his teeth cleaned).  His thyroid hormone levels were moderately elevated, and his heartbeat was faster than normal.  The teeth cleaning was postponed because the hyperthyroidism was placing a strain on his heart and that meant he could not safely undergo anesthesia.  Barney was started on Tapazole, 5 mg twice a day.  Shortly after starting Tapazole therapy, he started vomiting several times a day.  Since he is also diabetic, it is difficult to know if the vomiting was caused by the Tapezole, or if it was due to his diabetes not being well controlled, or due to some unknown reason. But vomiting is not something you want with a diabetic because you need to try to keep their food intake as consistent as possible.  Barney's thyroid hormone levels were not extremely high, so the vet thought it would be ok to decrease his Tapezol dose to 1/2 a tablet twice a day.  He showed an immediate improvement and the vomiting stopped.  A follow-up blood test was done, and his thyroid hormone levels were in the low-normal range.  Barney has continued on this low dose of Tapazole.  Since the hyperthyroidism increased metabolism and had an effect on the diabetes, during the time when his thyroid hormones were returning to normal, we monitored his blood glucose levels to be sure he was not getting too much insulin.



  • Pocket Companion to the Fourth Edition of Textbook of Veterinary Internal Medicine Stephen J. Ettinger.
  • The Cornell Book of Cats: A Comprehensive and Authoritative Medical Reference for Every Cat and Kitten. Mordecai Siegal (Editor), et al.1997.
  • The 5 Minute Veterinary Consult: Canine and Feline. Larry Tilly and Francis W.K. Smith, Jr. 1997.
  • Veterinary Drug Handbook.  Second Edition.  Donald C. Plumb.  1995.
  • The Well Cat Book Terri McGinnis, D.V.M. 1993.

This site is for information purposes only.  Please consult your veterinarian.