Hyperthyroidism is a common disease of older cats. It is usually caused by a benign tumor in the thyroid gland; very rarely, the tumor can be malignant (less than 2% of cases).
The thyroid gland is located in the neck and plays a very important role in regulating the body's metabolism. The hormones it releases increase the metabolic rate. The symptoms of hyperthyroidism result from having too much thyroid hormone, and consequently a very high metabolism.
A cat is at increased risk for hyperthyroidism with advancing age. There may be specific risk factors, but at present they are poorly understood. There does not appear to be a breed predisposition.
The typical cat with hyperthyroidism is middle aged or older. The most consistent finding with this disorder is weight loss secondary to the increased rate of metabolism. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! In more advanced disease the appetite falls off. Despite the early increased intake of food, most cats lose weight. Affected cats often drink a lot of water and frequently urinate. There may be periodic vomiting or diarrhea, and the hair coat may be unkempt.
Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the increased pumping pressure of the heart. Heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these problems are potentially reversible with appropriate treatment of the disease.
In most instances, diagnosis of this disease is relatively straightforward. The first step is to determine the blood level of one of the thyroid hormones, called thyroxine (or T4). Usually, the T4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the upper range of normal cats. When this occurs, a second test (T3 suppression test) is performed. If this is not diagnostic, a thyroid scan can be performed at a veterinary referral center or the T4 can be measured again in a few weeks.
Options for treatment
[For the less patient, click for a concise list of the pros and cons of the non-surgical treatments.]
Radioactive iodine. This is generally a curative treatment. The outcome following radiation therapy is usually excellent, and most cats have a very good chance of returning to a normal state of health. The radioactive iodine is given in a single injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment requires one week of hospitalization at a veterinary hospital licensed to administer radiation therapy. In Vancouver this is at the Northwest Nuclear Medicine for Animals facility near Granville Island. The cost of this treatment is around $1600, but it is usually curative. If the treatment does not work the first time, NNMA will retreat the cat at no charge.
A couple of follow-up checks are needed, one and three months after treatment. After that, no routine thyroid testing should be required. Possible complications include hypothyroidism (T4 level too low after treatment) and "unmasking" of kidney disease. This will be thoroughly discussed with you before you make a treatment decision.
Oral medication. Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Somewhere between 20 and 30 pecent of cats have reactions to the drug. Side-effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Methimazole does not destroy the abnormal thyroid tissue but rather prevents the production of excess thyroid hormone. Therefore, the drug must be given for the remainder of the cat's life. Periodic blood tests must be done to keep the dosage regulated, generally every 6 months. Cost of this treatment varies, but will average out to about $400 a year if there are no complications. Keep in mind that if your cat has an adverse response to the drug there will be medical bills associated with treating this.
Surgery. Surgical removal of the affected thyroid lobe(s) is also effective in some cases. If the disease involves both lobes of the thyroid gland, two surgeries may be required, depending on the surgeon's choice of procedures. In many cats, only one thyroid lobe is abnormal, so only one surgery is needed. Cost of surgery is approximately $1000-1400.
There are two main "issues" with surgery. The first is that the thyroid gland can be quite diffuse, and not just one neat package of glandular material. Surgery may not remove all of the abnormal tissue. The other issue is the parathyroid gland, which lies just beside the thyroid. It is tiny, and can be accidentally removed with the thyroid gland. It is vital in mineral metabolism, and removal can be life-threatening.
Recurrence of the disease is a possibility in any cat. It is uncommon after radioactive iodine therapy. When surgery is done, recurrence is possible if abnormal thyroid cells are left in the cat. The remaining cells will likely grow causing the disease to recur. However, this occurs less than 5% of the time and usually after 2-4 years. Another possibility for disease recurrence is that one lobe of the thyroid gland was normal at the time of surgery so it was not removed. Then, months or years later, it became abnormal.
There are no preventive measures to adopt, but middle-aged and geriatric cats should all receive a complete physical examination by a veterinarian every 6-12 months. Special attention should be given to thyroid enlargement and the typical clinical signs of hyperthyroidism.