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Educating Clients About Managing Feline Hyperthyroidism

Margie Scherk, DVM, DABVP, catsINK, Vancouver, Canada

January / February 2014

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When veterinarians tell clients that their beloved elderly cat has hyperthyroidism, they may be making a routine diagnosis, but the pronouncement likely is dramatic, frightening, and confusing for the clients. The explanation of the condition will influence clients’ short-term decisions, as well as their long-term management of the patient.

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If a client seems confused or distressed, it is important to acknowledge that is a normal response. They want, and need, to hear that their pet will be okay.

Veterinarians can convey that information with these answers to frequently asked questions:

Question: What is hyperthyroidism?

A: The term hyperthyroidism describes over-activity of the thyroid gland, which is located at the front of your cat’s neck below the larynx. The gland produces hormones that regulate metabolism—the rate at which cells use energy. In most cats, a benign growth causes the hyperactivity. In a few cases, a tumor that spreads to other tissues causes the hyperactivity but fortunately, even in that situation, the condition is treatable.

Q: Will my cat die from this disease?

A: Hyperthyroidism can be readily treated and is curable, depending on the type of treatment. However, if the condition  goes untreated and progresses, cats can die from heart failure or complications from emaciation or hypertension.

Q: Could I have prevented this? Did I do something wrong?

A: No. Hyperthyroidism is common in middle-aged to elderly cats. The disease is most often diagnosed in cats 10 to 13 years old.

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Q: What causes hyperthyroidism?

A: Veterinarians have known about this condition since 1979, but still do not know its cause. Multiple components are likely, such as a genetic predisposition, and dietary and/or environmental factors. Research has looked at iodine, soy, fish, and liver, as well as flame retardants used in fabrics and electronics. 

Q: What are the signs?

A: The most common clinical signs include weight loss despite an increased appetite; increased energy and activity, possibly to the point of hyperactivity; restlessness; and increased drinking and urinating. Some cats may have diarrhea, vomiting, or a poor hair coat. Some may seek, or avoid, heat. Some may show generalized weakness or a weak neck only, and changes in their voice. If the disease is advanced, cats may have labored breathing and breathe with their mouth open, which may indicate serious heart disease. Some cats may have a decreased appetite but be overweight—this less common presentation is called apathetic hyperthyroidism. 

Q: How is hyperthyroidism diagnosed?

A: A blood sample is taken to measure the level of a thyroid hormone called thyroxine, which is referred to as total T4. Various other tests may follow, including what are called a free T4 or a T3, to verify the diagnosis if the results of the T4 are not clear cut. Thyroid function or imaging tests, a chest radiograph (ie, an x-ray), or ultrasound of the heart (an echocardiogram) may be recommended. Your cat’s blood pressure should be measured because some cats are hypertensive (ie, have high blood pressure) when hyperthyroidism is diagnosed. A urine sample should be collected.

Because hyperthyroidism affects many organs, the blood sample and urinalysis will also be used to check liver and kidney function. These can determine if your cat has chronic kidney disease, which is a decline in kidney function commonly found in cats in the same age group. The radiograph or ultrasound will show if the disease has affected your cat’s heart. Blood pressure is checked because it is important to look for increased blood flow to the kidneys, which may hide age-related chronic changes. It is also important to continue to monitor the kidneys to detect and treat any decline in their function.

Q: How is hyperthyroidism treated?

A: An injection of radioiodine (called I-131) cures hyperthyroidism and is considered to be the gold standardtreatment. Some cats may need thyroid hormone supplements afterward, but re-treatment is rarely needed.

Surgery to remove the thyroid may also cure the disease, but more than one surgery may be needed. Anesthesia can be  complicated due to the heart’s increased sensitivity to anesthetic agents. Again, radioiodine treatment is preferred because anesthesia and surgery risks are avoided.   

Medication or diet can be used to control, rather than cure, the condition. Methimazole or carbimazole decrease the circulating levels of T4; if these drugs are used, your cat’s T4 blood level should be checked every 10 to 14 days until it falls within normal range or slightly higher. Approximately 15% of cats experience side effects that can be reversed by decreasing the dosage or discontinuing the drug. Methimazole can be given as a pill, or in gel form that is rubbed into the skin of your cat’s ear.

It is also possible to treat a patient using a special diet with extremely low iodine levels that will decrease his (or her) T4 levels. It is critical that the patient be given no other food, treats, or supplements, or anything other than water to drink with this diet.

Q: Will my cat need monitoring after the thyroid is controlled?

A: Unless radioiodine is given or the thyroid is surgically removed, the tumor will continue to grow and your cat’s frequent T4 measurements should be taken frequently to find the correct dosage to control the condition. If the tumor does continue to grow, your cat’s T4 values and kidney enzymes should also be measured regularly to adjust the drug dosage. Your cat’s liver enzymes and a blood cell count should be monitored because of the small chance of liver or bone marrow toxicity from drug therapy. If he (or she) has radioiodine treatment or surgery, these tests need to be run to check if he (or she) needs thyroid supplements and to check kidney function.

Q: How are hypertension and heart and kidney disease treated?

A: Hypertension is treated with a medication called amlodipine to lower blood pressure. If your cat develops heart or kidney disease, treatment should be individualized specifically for him (or her).

Q: Where can I get more information?

A: The Animal Endocrine Clinic has an excellent website.

Reassuring clients and educating them about their cat’s hyperthyroidism is an opportunity for veterinarians and the team to bond with their clients and grow their trust by providing the information they need with compassion and empathy.

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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