Hyperthyroidism in Cats
FACT SHEET: Hyperthyroidism in Cats
Hyperthyroidism is an extremely common condition in cats, especially in cats 8 years of age or older. Most cats with this disease have excellent-to-ravenous appetites and weight loss. Variably, afflicted cats may be nervous, restless, or anxious; they may seek cool areas to stay; they may have vomiting and/or diarrhea; they can become weak; they drink and/or urinate excessively; and some have episodes of heavy breathing. The diagnosis for most (not all) cats is straight-forward by testing the thyroid hormone levels in the blood. This is usually a relatively inexpensive simple blood test. Suspicion of this condition is based on a veterinarian learning of typical owner-perceived problems together with feeling an enlarged thyroid lobe on physical examination.
Cats normally have 2 thyroid lobes, one located on each side of the trachea (windpipe) in the neck. Normally, neither thyroid lobe can be felt, even by trained veterinarians. More than 80% of afflicted cats have a condition known as 'hyperplasia' of the thyroids. This results in excessive function of both thyroid lobes and enlargement of both lobes. However, both lobes cannot always be felt, even if both are functioning excessively. About 15% of hyperthyroid cats have a benign tumor (an adenoma) and in this situation, only one of the 2 lobes is involved.
There are 3 primary methods of treating hyperthyroid cats, each is quite effective. One treatment method is with oral medication (tapazole, methimazole) which blocks the production of thyroid hormone. Surgery is another option, whereby the surgeon removes all abnormal thyroid tissue. Radioactive iodine is the third treatment. Iodine is the main 'building block' for creating thyroid hormone. Most of the iodine consumed by cats, dogs and people is concentrated in the thyroid. The thyroid then uses as much iodine as it needs and excess iodine is then eliminated from the body. If the iodine has a certain type of radioactivity, that radioactivity kills the abnormal cells. Oral therapy can be given to any hyperthyroid cat. Surgery is the best treatment option for cats with an adenoma, while radioactive iodine is the best treatment for cats with both lobes involved (hyperplasia).
UC Davis recommends a thyroid scan prior to treating any hyperthyroid cat. Simply put, the scan is the most reliable tool for determining if a cat has an adenoma (one lobe involved) or hyperplasia (2 lobes involved). Further, while less than 5% of hyperthyroid cats have cancer (malignancy), the thyroid scan is the best tool to identify this uncommon form of the disease, since these cats have more than 2 thyroid lobes visible. Now, the scan is not a perfect test, but it is quite reliable.
Pre-Treatment with Tapazole (methimazole)
We have learned over the years that some cats with hyperthyroidism also have kidney failure, but their kidney failure is hidden by the thyroid condition. Therefore, before embarking on a permanent treatment (surgery or radioactive iodine) that might cure the hyperthyroidism but cause kidney failure, we typically want to see what happens to kidney function when the hyperthyroidism is treated completely. If kidney failure does not occur, permanent treatment can be employed. If kidney failure does occur, it is usually more worrisome and life-threatening than hyperthyroidism. In those cats, we usually do not employ surgery or radioactive iodine.
The first appointment you have at the VMTH will be for evaluation and possible thyroid scan. This should be scheduled on a Monday or Wednesday with the SA Internal Medicine Service. Please plan for your pet to stay with us for 2 nights if a thyroid scan is recommended. The cost associated with this first visit for Medicine workup with thyroid scan is $600-1,000.
The actual Radioactive Iodine Therapy will not happen at this first visit. You will need to bring your pet back on a Monday and leave them with us until Friday for the treatment. The cost associated with Radioactive Iodine therapy(including hospitalization) is $1300-2,000.