Published by: Ingrid King. Last Updated on: October 31, 2022 by Crystal Uys

Hyperthyroidism is a common disease that typically affects middle-aged and older cats.  It is caused by an excess production of thyroid hormones, which are produced by the thyroid gland, located inside the cat’s neck.  Thyroid hormones affect nearly all organs, which is why thyroid disease can sometimes cause secondary problems such as hypertension, heart and kidney disease. 

What causes hyperthyroidism?

The most common cause is an increase in the number of cells in the thyroid gland.  Groups of these abnormal cells form small nodules called adenomas on the gland.  Most of these adenomas are formed by non-cancerous cells, only a very small percentage of hyperthyroidism is caused by malignant tumors.

More recently, there has been speculation on a possible link of an increase in thyroid disease in cats and the coating used on cat food cans.  Another theory is that flame retardants used in furniture and carpeting may be linked to hyperthyroidism in cats.

What are the signs of hperthyroidism?

Afflicted cats often develop a variety of signs, and some of them can be subtle.  The most common signs are weight loss, increased appetite without weight gain, and increased thirst and urination.  Hyperthyroidism can also cause vomiting, diarrhea, and hyper-activity.  The haircoat may become matted and dull.  Some cats will begin to vocalize more frequently.  Rapid heart rates are common, and cats can also present with heart murmurs and high blood pressure.

How is hyperthyroidism diagnosed?

You cat’s veterinarian will perform a physical exam and palpate externally alongside the trachea with thumb and forefinger to feel for any enlargement of the thyroid gland.  Heart rate and blood pressure will be checked, and a complete blood chemistry will be run.  Most hyperthyroid cats will have elevated levels of the thyroid hormone T4 in their blood stream.  However, sometimes a cat with concurrent kidney, heart or gastrointestinal disease may have normal T4 levels.  If other symptoms and exam findings point to hyperthyroidism, your veterinarian may order additional testing to arrive at a diagnosis.

How is hyperthyroidism treated?

There are currently three treatment options:  medication, surgery, and radioactive iodine therapy.  Each option comes with advantages and disadvantages, and you should carefully weigh all options and make the best decision for your cat and your lifestyle in conjunction with your veterinarian.

Medication

Drug therapy, using a drug called methimazole (Tapazole), controls, but does not cure the disease.   It is typically given twice a day in either pill form or as a transdermal gel that is rubbed on the inside of the cat’s ear.  Methimazole therapy will be required for the rest of the cat’s life.   While some cats tolerate the drug well, it can have serious side effects including elevation of liver enzymes, low white blood cell counts, low platelet counts, itchiness of the face, and gastrointestinal problems, including vomiting and loss of appetite. If these signs occur, the medication has to be discontinued and other treatment pursued.

Surgery

Surgical removal of the thyroid gland is an option, although many hyperthyroid cats won’t be good candidates for surgery due to the anesthetic risk caused by their elevated heart rates.  Even though removal of the thyroid gland is a fairly straightfoward procedure, it should only be done by an experienced surgeon, since there are potentially serious complications, including damage to the parathyroid glands, which lie close to or within the thyroid glands and are crucial in maintaining stable blood-calcium levels.

Radioactive Iodine

Radioactive Iodine, also called I-131, is the gold standard for treating hyperthyroid cats.  It involves a one-time injection of radioactive iodine under the skin.  The radioactive iodine will destroy the abnormal thyroid tissue but does not damage the surrounding tissue or the parathyroid gland.  The cat will have to remain hospitalized for a specified period of time (typically 3-10 days, depending on geographical location, the length of the stay is regulated on the state level).  It will be released with some special care instructions, such as limiting contact with the cat and special disposal of urine and feces for a few days following treatments.  The treatment is only available at special facilities that are typically found at large veterinary referral centers, and is somewhat costly, but it is curative, and needs to be weighed against the cost of lifelong medication.

Regardless of which treatment is chosen, unless there are other, underlying diseases complicating things, treatment is usually successful and most cats will lead normal, healthy lives.

The photo above is of Amber, taken the day she went for her radioactive iodine treatment at Radiocat in Springfield, VA.

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