Hyperthyroidism is a very common condition in cats over the age of eight. The thyroid gland is located in the neck of your cat and produces thyroid hormone, which controls your pet’s rate of metabolism. With hyperthyroidism an enlarged thyroid gland produces excess thyroid hormone causing your cat’s metabolic rate to skyrocket. All older cats are at risk of developing this disease.
Symptoms of hyperthyroidism can include weight loss, increased appetite, increased water intake, increased urination, increased vocalization, increased heart rate, vomiting and diarrhea. The increased metabolic rate of cats with hyperthyroidism causes the heart to pump faster and harder often resulting in high blood pressure. Extremely high blood pressure can cause irreversible kidney damage as well as sudden onset blindness caused by retinal hemorrhage or detachment. Additionally the increased cardiac output can result in enlargement and thickening of the heart. In some cases a heart murmur can arise as a result.
Diagnosing hyperthyroidism is done with a blood test. The primary thyroid hormone produced by the thyroid gland is called thyroxin. A blood test called a T4 tests the level of thyroxin in the blood. The T4 test is included in our routine blood work that is sent off to the lab. Aside from a T4 it is also important to perform full blood work including a Complete Blood Count and Chemistry whenever possible when trying to diagnose hyperthyroidism. Before beginning treatment for hyperthyroidism it is important to ensure that all other organ systems- especially the kidneys are functioning properly. Kidney disease is relatively common in aging cats and can be greatly affected by the treatment of hyperthyroidism. In cats with elevated kidney values it is important to realize that treating their hyperthyroidism will decrease their metabolism, thus decreasing the rate of blood being filtered through the kidneys. Decreasing the rate of filtration will cause levels of toxins in the blood to increase and can result in increased symptoms of renal disease.
There are several appropriate options to treat hyperthyroidism. He best and most successful treatment option is radioactive iodine therapy. This therapy is performed at specialized facilities and requires several tests such as full blood work, radiographs, urine testing and blood pressure monitoring before it can be done. Radioactive iodine therapy consists of an injection of radioactive iodine that destroys the abnormal thyroid tissue exclusively. This treatment has a great success rate and often requires at least several days of hospitalization.
Surgical removal of the thyroid gland is another older option for treating hyperthyroidism. Since surgical removal of the gland requires full anesthesia there is always a risk in sedating an older animal. Treatment with a medication that blocks the production of excess thyroid hormone for several weeks prior to surgery can help to decrease this risk by normalizing blood pressure and heart rate and encouraging weight gain.
Most cats who develop hyperthyroidism are treated with an oral medication called Methimazole. This medication is available in a tablet or a chewable form and is usually administered 1-2 times daily. Methimazole works by blocking the production of excess thyroid hormones, and must be given for the remainder of your cat’s life. The initial dosage of Methimazole should be given for 30 days and then a T4 should be rechecked in the clinic. Further adjustments may be necessary and the T4 should be tested every 30 days until the thyroxin levels fall within the recommended range. Once the correct dosage of Methimazole has been determined a T4 should be performed every 6 months. As cats age the dosage may need periodic adjustment, so these semi-annual tests are extremely important in order to ensure that your cat continues to get the best treatment possible.
Methimazole works well in almost all cats but has been known to cause significant side effects in less than 20% of patients including itching, vomiting, lethargy, anorexia, fever, and liver damage. It is also important to note that although treatment with Methimazole is the most economical choice initially it is quite possible that your cat will live several years and the cost of the medication and blood monitoring will end up equaling the cost of radioactive iodine therapy.
The newest treatment for hyperthyroidism is actually a diet called y/d. This food is manufactured by Hills and is a prescription diet available in both a wet and dry form that is limited in iodine. It is important that you are able to feed your cat only this food and that he/she does not have access to anything else to eat. If your cat is able to eat other food that is not limited in iodine the diet will not work. Periodic T4 testing is required while on y/d food.