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Hyperthyroidism in Cats

Written by Rhonda Downie, DVM.

Older cats are prone to a condition called hyperthyroid disease. This occurs when a benign overgrowth or tumor starts in the thyroid gland. The thyroid gland is located in the neck of the cats. The average age of onset is 12 years old. The cause is unknown.

When the thyroid gland produces too much hormone it can cause a variety of symptoms including: 1) weight loss 2) a ravenous appetite 3) increased thirst and urination 4) vomiting 5) diarrhea or excess stool 6) unkempt fur 7)yowling

The weight loss can be severe even when the cats are eating 2-3 times their normal amount of calories. These cats are very hungry because their metabolism is very high from the excessive thyroid hormone.


Many of the cats will have excessive urination and thirst. This can be complicated because many of these older cats can also have kidney disease which can have the same symptoms.


Excessive vomiting can also occur because not only are they eating alot of food, but the hyperthyroid disease can also cause an inflamed condition of the intestines called inflammatory bowel disease. Diarrhea or excessive stool can also go along with this condition.


The cats fur will usually look rumpled or unkempt.


The most common complaint from owners is that the cat is yowling or meowing more often and loudly.


A serious complication of hyperthyroid disease is heart disease and or hypertension. This complication usually resolved with treatment. The heart muscle can become thickened and a murmur can be heard in some cats.The diagnosis is done easily with a simple blood test that shows too much thyroid hormone. A radioactive scan to can also be done at the University of Minnesota to diagnose this disease.There are three ways to treat hyperthyroid disease.

1) surgery to remove the benign tumors of the thyroid gland.
2) radioactive iodine treatment at the University of Minnesota
3) daily medication to block the overproduction of thyroid hormone.


Surgery must be done with an experienced surgeon. The thyroid glands are removed but the parathyroid glands which are inbedded in the tissue must be spared to prevent a life-threatening condition involving the regulation of calcium. The cat will then have to take life long thyroid supplements.


Radioactive treatment, which is done at the university of Minnesota, is very safe. It is a complete cure as it destroys the abnormal thyroid tissue, but leaves the normal tissue intact. No treatment is needed afterwards. It is very important for the cat to not have kidney disease or the treatment will make that disease worse.


The most common and economical treatment is to give the drug methimazole. This medication blocks the excess thyroid hormone. Follow-up blood testing is needed to make the blood level of thyroid hormone is now normal. This medication comes in a tablet, liquid, and a topical creme that is put on the ear.


The prognosis for this disease is excellent. The cats can live a symptom free life with treatment. There is no known way to prevent it.

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