Hyperthyroidism in cats

Overview

Hyperthyroidism is the most common endocrine disease in American cats.  It is a medical condition usually in older cats where the thyroid gland produces too much thyroid hormone. The thyroid gland is made up of two lobes located on either side of the trachea (windpipe). The thyroid gland is responsible for controlling the body’s metabolism as well as playing a part in every system in the body. If left untreated a cat can develop a heart condition known as hypertrophic cardiomyopathy where the heart muscle becomes severely thickened and ultimately results in death.  Hyperthyroidism can also have negative effects on the kidneys and eyes. 

The most common cause of hyperthyroidism is a non-cancerous growth that develops on the thyroid gland. The average age of a cat that develops this condition is about 12 years old, but it can occur as early as 6 years of age.  No one is quite sure of what causes hyperthyroidism, but there is recent evidence that suggests it may be caused by the fire retardants that are put on the furniture and rugs in our homes. 

Diagnosis is made by the veterinarian through physical exam and blood tests.

Symptoms

• Weight loss
• Increased appetite, often ravenous eating
• Increased activity/behavior changes 
• Increased drinking/urinating
• Poor grooming
• Vomiting, usually from eating too fast
• Diarrhea
• Increased stool volume
• Increased heart rate

Treatment

The choices for treatment depend on the age and general overall health of your cat. Radioiodine therapy (I-131)is a very effective method of treatment with few side affects.  It does not require anesthesia or surgery, but your cat will have to stay in the hospital after receiving the radioactive treatment for a period of days.  This procedure destroys the over functioning thyroid tissue with out damaging the healthy tissue in the area.  Normal thyroid function returns in about a month and requires no medication.  This procedure is usually done at a specialist and requires hospitalization making it somewhat more expensive than other treatments.

There is also an antithyroid drug available called methimazole which is given once or twice daily.  Most cats do well with this therapy but there are some who cannot tolerate the drug.  This is the least expensive avenue initially but requires the owner to medicate the cat twice a day which can be very hard for hyperthyroid cats because they are often very disagreeable.  The down side is that the cat will require this medication for the rest of its life because the medication does not do anything to actually destroy the overactive tissue.  The time, energy, and money required to dose your cat every day for the rest of his life will add up!  Methimazole is usually used for a period of time before I-131 treatment or before surgery is attempted.  This is done as a way of making sure your cats kidneys are still healthy and to lower the heartrate to a safe level to handle anesthesia.   

If your cat is a good candidate for surgery, your vet may recommend a procedure called a thyroidectomy.  This is a term for the removal of the thyroid gland.  Your cat would most likely be put on methimazole for at least two weeks prior to the procedure to stabilize the heart rate and make sure the kidneys are functioning properly.  After surgery your cat should not require any daily medication.  If not done carefully, there can be nerve damage to the surrounding area during surgery, so there is a risk involved.  This surgery is potentially dangerous if a gland known as the parathyroid gland is damaged during the procedure.  The parathyroid gland is responsible for the regulation of calcium in the body and without it, your cats calcium level can get critically low.  If this is not corrected immeidately, it can ultimately result in death.  Most surgeons who do this procedure routinely have methods to ensure that this does not happen.  Feel free to question your veterinarian and their surgical skill before you elect to have your cat operated on. 

The Future

If your cat is treated for hyperthyroidism, it is possible that he can become hyperthyroid again in the future.  Methimazole does not correct hyperthyroidism and the symptoms will return immediately if the medication is discontinued.  I-131 treatment does not kill off all the thyroid tissue and the remaining tissue can become hyperactive in the future.  If any tissue is left during a thyroidectomy, it too, can become hyperactive in the future.  After any of these procedures, your veterinarian will want to monitor your cat's thyroid levels periodically to make sure they do not get too high again.  Once the thyroid levels are normal after treatment, any of the changes to the heart muscle should resolve over time.  Resolution of hyperthyroidism may unmask underlying kidney disease and your veterinarian will want to monitor this as well.