Visiting Veterinarian

Treating thyroid conditions

By Michele Gerhard Jasny V.M.D. - March 15, 2007

Last week a woman called to tell me that her geriatric cat had recently been diagnosed with hyperthyroidism. Her veterinarian was suggesting surgery. The owner was worried. Surgery sounded risky. Radioactive iodine therapy sounded scary and expensive. She had heard she could just give her cat pills.

Would I give her my opinion? What did I think about surgery? Well, I once heard it said that the only thing two veterinarians can agree on is that the third veterinarian is wrong. It's true that different doctors have very different ideas about treating hyperthyroidism. In order to understand the pros and cons of the various options, we have to start by understanding the basics of the disease

Take your cat, Speedy. Her thyroid gland consists of two tiny lobes lying along either side of her windpipe. It secretes hormones that control her basic metabolic rate. In a normal individual, the pituitary gland in the brain "tells" the thyroid gland in the neck when to make more hormones, and when to make less, keeping the body in a fairly stable metabolic state. In feline hyperthyroidism, little nodules in the thyroid ignore the pituitary and randomly secrete additional hormones, thus raising the metabolic rate higher and higher. Speedy's heart goes faster. She becomes hungrier and hungrier, but because her metabolism is cranked up so high, no matter how much she eats, she doesn't gain weight. In fact, she loses weight. She may become hyperexcitable or exhibit abnormal behavior, acting restless and irritable. Occasionally a cat with hyperthyroidism will present with just the opposite signs, being depressed, weak, and anorectic.

Hyperthyroidism is one of the most common endocrine abnormalities in older cats. The nodules themselves are usually benign, but the excess hormone can eventually be lethal. Although Speedy may have a nodule that Doc can feel, a percentage of hyperthyroid cats do not have palpable masses. In some cases, excess thyroid hormone is being produced by "ectopic hyperplastic thyroid tissue" or EHTT. This is tissue that is just like a piece of thyroid, only it's in some weird place, like inside Speedy's chest. The only way to locate EHTT is with a test called nuclear scintigraphy. You'll see later why EHTT is important in deciding how to treat Speedy.

Diagnosis is usually straightforward. Along with her physical examination, your veterinarian will measure one or two types of thyroid hormones as well as other basic blood work. If thyroid levels are above normal, that's a definitive diagnosis. In a few cases, additional tests may be needed to confirm your veterinarian's suspicions, as the "normal" range is not that cut and dried. In an older cat exhibiting typical symptoms, many specialists say that any result in the upper half of the "normal" range is consistent with a diagnosis of hyperthyroidism.

Three options for treatment

There are three basic approaches to treatment: medicine, surgery, and radioactive iodine therapy. Without a doubt, radioactive iodine therapy is the gold standard. After appropriate preliminary care to assess whether Speedy has concurrent kidney or cardiac disease, she goes to a special facility and receives a single injection of an isotope called I-131 that selectively goes to abnormal thyroid tissue and destroys it, leaving normal tissue intact. Cost varies, but is generally around $1,300. I-131 is non-invasive. There is no surgery involved. Therefore there is generally no anesthesia required, always a good thing, but especially in these senior citizens. I-131 is virtually 100 percent effective. It will destroy palpable nodules in the thyroid and EHTT. Speedy goes home with a few weeks of special instructions as the last bits of the I-131 are excreted from her body, a bit of follow-up blood work, and then she's considered cured.

Medical treatment consists of giving daily medication that suppresses excess hormone production. It is a way to manage the disease, but not to cure it. A small percentage of cats have adverse side effects and cannot tolerate the drugs. The drugs are relatively expensive and need to be given lifelong, so the cost of medication and monitoring can rapidly add up. If Speedy lives more than a year or two, medical management may well exceed the cost of just doing the radioactive iodine and being done with it. However, for many owners, especially those with exceedingly ancient felines, the jaunt off-Island and the up-front expense of the I-131 therapy are daunting. Also cats with chronic kidney disease are not good candidates for either I-131 or surgery and are better off managed medically.

Surgery involves removing the abnormal thyroid tissue. The first concern is how to determine what to remove. Remember the thyroid has two separate lobes. In about one third of cases, one lobe is obviously abnormal, and the second is not. Surgically removing the abnormal lobe cures Speedy and has a relatively low risk of complications. It is a quick and fairly simple surgery. One of the problems, however, is that without doing the nuclear scintigraphy, you can't be absolutely sure that second lobe is really okay. Nor can you know whether she has any EHTT. If you remove the visibly abnormal lobe but leave behind and abnormal tissue or EHTT, Speedy will not be cured. In two-thirds of cases, both lobes are abnormal and should be removed. Again, without nuclear scintigraphy, you can't know for sure what tissue is normal and what is not.

Risks of surgery

Okay, you say, why not just remove both lobes completely? This can occasionally pose problems. Sitting right on the thyroid gland are the very teeny parathyroid glands that control calcium homeostasis. If the surgeon inadvertently removes them or badly damages them or their blood supply, Speedy can get into trouble. Proper calcium regulation is necessary for her muscles to work properly. This includes her heart muscle. If Speedy has both thyroid lobes removed, she should have her blood calcium carefully monitored for about a week. Occasionally a patient will have a crisis post-surgically, related to low calcium and may seizure, or rarely die. Cats who have had parathyroid damage may recover with just a few days of calcium and Vitamin D supplementation, or they may require lifelong supplements.

There are other risks. First is the anesthetic risk inherent in any such procedure in a senior citizen. Other complications can include damage to the laryngeal nerve or the sympathetic nerve trunk resulting respectively in a voice change or a neurological abnormality of the eye called Horner's syndrome. Neither of these is life-threatening, but owners should be aware of the potential problems. Some cats may become hypothyroid after surgery. In other words, if all the thyroid is removed, now they have too little hormone. These cats need to be given thyroid supplements daily, sometimes for the rest of their lives. The risk of recurrence of hyperthyroidism is also much higher than with I-131 treatment.

In April 2006 in Veterinary Surgery: the official journal of the American College of Veterinary Surgeons, an article was published entitled "Results of thyroidectomy in 101 cats with hyperthyroidism" by Naan, et al. This survey presented the following statistics. About 90 percent of cats had both lobes involved. One cat died of laryngeal spasm and one cat died postoperatively of unknown causes. About 5 percent had a temporary calcium crisis. About 5 percent experienced a recurrence. The authors summarized their findings as follows: "Complications were uncommon after thyroidectomy performed by an experienced surgeon when combined with an anesthetic regimen associated with minimal adverse cardiovascular effects . . . [It] is an effective treatment for hyperthyroid cats when radioactive iodine therapy is not available. Preoperative thyroid scintigraphy is advised. Surgery is not recommended when EHTT is present, because of a higher chance of developing recurrent disease."

That about sums up my feelings. If Speedy's kidneys and heart are fine, and the surgeon is experienced, surgery is a fairly low risk procedure with a good chance of being curative, especially if she has nuclear scintigraphy first. But in my opinion, if you're taking Speedy off-Island and spending the money for nuclear scintigraphy, why not skip it and have radioactive iodine? Don't be spooked by the word "radioactive." I-131 is the safest, most effective option. If you skip scintigraphy and go right for surgery, remember there is a small but real chance of complications and/or recurrence. Finally, any treatment, surgical, medical, or I-131, may unmask underlying kidney disease leading to kidney failure.

So, what did I tell the woman who called for advice last week? I encouraged her to talk to her regular veterinarian about the pros and cons and to voice her concerns. Without my crystal ball, that was the best advice I could offer.