Hyperthyroid Disease in Cats

Hyperthyroidism is one of the most common diseases of the middle-aged and older cats. It is a disorder that ultimately affects many of the body systems. It is caused by an increase in the amount of thyroid hormones produced by enlarged thyroid glands. First documented 30 years ago, the actual cause of the disease remains a mystery. In most cases, the enlargement in the thyroid gland is caused by a non-malignant tumor called an adenoma. In very rare cases, a malignant form of this disease is seen. The thyroid glands are located in the front of the neck on each side of the trachea (windpipe). Normally, they are tiny, about ¼ inch long, and difficult to feel through the skin. If the glands begin to enlarge, the veterinarian may be able to feel them. Hyperthyroidism is diagnosed by checking levels of the thyroid hormone in the blood. Since these levels can fluctuate daily, sometimes repeat testing or special thyroid function testing may be necessary for diagnosis. The most common symptoms of this disease include weight loss, increased appetite, increased thirst, restlessness, unkempt hair with excessive shedding and matting, vomiting and/or diarrhea (although these symptoms are often sporadic). Because of the effects of the thyroid hormone levels on the heart, these patients have a fast heart rate, and may have a heart murmur, high blood pressure, difficulty breathing, and other heart problems. It is important to note that not all of these symptoms may be present in every cat. Therefore, any middle-aged to older cat that presents with any of the above symptoms should be screened for hyperthyroidism. Once hyperthyroidism has been confirmed, there are several treatment options. They include treatment with radioactive iodine, surgical removal of the gland, and treatment with anti-thyroid medications. The initial choice of treatment is often guided by concern about the patient’s general health status. The veterinarian will want to monitor the kidney, liver and heart functions carefully before, during and after therapy.

For hyperthyroid cats that have no other problems, radioactive iodine treatment or surgery is often recommended. Both these options may provide a cure for hyperthyroidism and avoid the need for life-long administration of medications.There are effective anti-thyroid medications available, the most common medication being methimazole (Tapazole). Approximately 15% of patients will experience side effects when taking methimazole. These may range from poor appetite, vomiting, lethargy and skin rashes to the more serious problems such as bone marrow suppression and liver toxicity. In most cats, the side effects are mild and do not interfere with continued treatment. However, periodic monitoring will be necessary while the cat is on the medication.Methimazole works by inhibiting thyroid hormone production by blocking iodine incorporation into the hormone. It is important to understand that methimazole does not “cure” hyperthyroidism, rather it controls the symptoms by lowering hormone levels. Thyroid hormone levels will usually reach normal levels in 2-3 weeks after starting therapy, and it will need to be given daily, probably for life, to maintain remission. It is very important that this medication be given regularly because the hyperthyroid condition will return when it is stopped.

Although hyperthyroidism is a very serious disease in cats, it does have several very effective treatments. As with any medical condition, early detection can increase the chances of successful treatment. Geriatric cats (those over 7 years of age) can have periodic blood testing to screen for hyperthyroidism as well as other health problems. With proper treatment, most cats with hyperthyroidism can live a normal, high quality life.