Feline hyperthyroidism is a relatively common disease, typically affecting cats over 10 years of age.  It is usually caused by a benign tumor of one or both thyroid glands. These tumors produce excess thyroid hormone, which speeds up the cat's metabolism, causing weight loss. Hyperthyroidism, although common today, was not often seen in cats before the 1980's  The cause of this change in prevalence is not known, but there are studies ongoing to try to pinpoint the reason for the shift.

Symptoms

Hyperthyroidism is a disease that most commonly affects cats older than 10 years of age. Affected cats are usually underweight, despite a ravenous appetite. Often, owners will report an increase or change in vocalization habits. Due to the increased metabolism caused by excess thyroid hormone, affected cats will commonly have heart disease, accompanied by a heart murmur. A small thyroid "slip" or nodule can often be palpated on either side of the cat's neck during physical examination. This is the enlarged thyroid gland.

Diagnosis

Diagnosis of hyperthyroidism is based largely on clinical signs and blood testing. Once the veterinarian suspects hyperthyroidism, based on the history of weight loss and increased appetite, physical exam findings, or other clinical signs, a simple blood test will be submitted to check the cat's thyroid hormone level, or T4  In most cases, the diagnosis is straightforward. However, in some cats with clinical symptoms of hyperthyroidism but a borderline T4 level, advanced testing may be necessary. If your cat has had routine screening senior bloodwork, a T4 level was probably part of this blood testing to screen for early hyperthyroidism.

Treatment

Hyperthyroidism can be a devastating disease if not treated properly  Luckily, there are several very effective options for treatment. The "gold standard" treatment for hyperthyroidism in cats is radioactive iodine therapy. This treatment is also commonly used for people with hyperthyroidism. 

Radioiodine therapy involves the administration of an oral or injectable dose of radioactive iodine. The iodine is concentrated into the thyroid gland, where the radiation kills off the overactive thyroid tissue. Usually, enough normal thyroid tissue is spared so that treated cats don't need to take any thyroid supplementation after treatment. While very safe and effective, radioiodine therapy is expensive - usually $1000-1200. However, when compared to perhaps several years of medication and monitoring bloodwork, the cost of treatment is reasonable  Another drawback to the treatment for some pet owners is the need to leave their cat at the treatment center for several days for treatment. This is necessary due to the radiation given off by the cat's body after treatment with the radioactive iodine. After successful treatment with radioactive iodine, most cats experience a complete cure and require no additional medication or monitoring.

Another treatment for hyperthyroidism is surgery to remove the enlarged thyroid gland. Due to the increasing prevalence of radioiodine treatment centers, surgery is performed less commonly today than in the past. Surgery is usually curative, although there is a risk of immediate postoperative complications, and cats need to be monitored very carefully in the hospital for several days after treatment. Most cats do not require ongoing medication after surgery, although hyperthyroidism can relapse if the cat develops another tumor in the opposite thyroid gland.

The most common treatment for hyperthyroidism is chronic oral medication to prevent the thyroid gland from producing too much thyroid hormone. The medication, called methimazole, is available in the form of a pill, and usually needs to be given twice daily  A blood test is performed 1 month after starting the  medication, and then every 6 months thereafter to monitor treatment. For cats that don't take pills well, the medication may be compounded into a liquid form or into a transdermal gel, which is rubbed into the skin in the cat's ear.

Monitoring

Once a treatment method is chosen, appropriate monitoring can be instituted. After surgery or radioiodine therapy, chronic monitoring is usually not necessary. Blood tests are usually performed 1 month after the procedure to ensure that the cat is producing adequate thyroid hormone.

For cats taking methimazole, a physical examination and blood test are necessary every 6 months to ensure safe and effective treatment. Some cats can develop kidney disease after treatment for hyperthyroidism, so monitoring is important to detect problems early.

With early diagnosis, appropriate treatment and monitoring, cats with hyperthyroidism can be successfully managed and live normal, healthy lives.

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