It was 1979. My second year of veterinary school. A call came to our lecture hall asking the professor to cut class short and bring all the students to the pathology lab to see an unusual case. We closed our notebooks, trooped out into the old, cobblestoned quad, and across to Pathology where Dr. Medved ruled. As we crowded into the small laboratory, all 104 of us, Dr. Medved beckoned us to a stainless steel necropsy table on which lay a very skinny, very deceased white cat.
“This 12-year-old spayed female cat presented to its veterinarian with a history of polyphagia and weight loss,” he intoned. (Polyphagia means excessive appetite.) He then proceeded to grill us on possible diagnoses. One student suggested diabetes. Another offered maldigestion or malabsorption. Intestinal lymphoma, a third proposed.
Dr. Medved fairly crowed with delight. “No, no, and no! Come. Come. Look here.” As we jockeyed for position around the table, I finally procured a spot close enough to see the tiny pink nodule on the unfortunate animal’s thyroid gland.
Feline hyperthyroidism was first described in the veterinary literature in 1979, hence Dr. Medved’s excitement and desire to share this rare finding with us that day 30-odd years ago. But in 2011, hyperthyroidism has become one of the most common endocrine abnormalities in middle-aged cats in the United States. Some studies have implicated canned cat food, particularly fish, liver, or giblet flavors, or exposure to cat litter as increasing the risk, but no definitive underlying cause has been proven.
Affected cats have enlarged thyroid glands, which secrete excessive hormone, increasing the basal metabolic rate, causing the typical presentation of increased appetite combined with weight loss. Many also exhibit increased thirst and urination, and behavioral changes which may include hyperactivity, irritability, aggressiveness, poor grooming habits, or personality changes. One of the first cases I personally diagnosed in the early 1980s presented with the sole complaint that the cat had taken to sitting in puddles in the driveway. That odd behavior, combined with the cat’s badly matted coat, led me to suggest testing for hyperthyroidism.
There are three traditional treatments — surgery, medical therapy, and radioactive iodine. Surgical removal of affected glands is still done by some veterinarians today. There are several possible significant complications. Incomplete removal of abnormal tissue may lead to recurrent disease. Inadvertent removal of the parathyroid glands may cause a potentially life-threatening drop in blood calcium levels. Nerve damage may result in laryngeal paralysis.
Medical therapy works by blocking hormone production. Side effects may include allergic reactions, facial itching, vomiting, loss of appetite, and, rarely, life-threatening bone marrow suppression. But the biggest negative is that the medical approach is a treatment, not a cure, and thus requires giving medication to the cat usually twice a day for the rest of its life. Both owner and cats often tire of this. Luckily the small, relatively tasteless pills can often be delivered mixed in a tasty treat. It is also available in a gel that can be rubbed into the ear to absorb through the skin.
Finally there’s radioactive iodine therapy, considered the gold standard. A fairly straightforward procedure with close to 100 percent success, the main difficulty, especially for diehard Islanders who hate schlepping off The Rock, is that treatment is available only at specially licensed facilities. A single injection of radioactive iodine destroys all abnormal thyroid tissue. After three to five days hospitalization, kitty goes home. There are special precautions to observe for a few weeks, then you’re done.
Now a new product has hit the market for hyperthyroid cats. Food. That’s right. Food. Hill’s Pet Nutrition has recently introduced a prescription veterinary diet called Y/D. Available in dry and canned, Y/D contains extremely low levels of iodine — lower than the recommended daily amount for normal cats. In a series of studies, hyperthyroid cats were fed iodine-restricted diets and their thyroid hormone levels monitored. When fed exclusively, these extremely low-iodine diets eventually resulted in normalization of hormone levels in hyperthyroid cats. The researchers tried different levels of restriction. Hills then designed Y/D to contain the maximum amount of iodine still low enough to revert most hyperthyroid cats to normal. Without medication. Without surgery. Without radioactive iodine.
Sounds too good to be true, doesn’t it? The new product has aroused controversy among veterinarians. The studies done were on very small numbers of cats. Iodine is necessary for thyroid hormone production but probably plays other physiological roles as well. All the long-term effects of feeding such a diet have not been determined. And Y/D only works if fed exclusively, without any other foods or treats. Since everyone agrees it is not an appropriate diet for normal, healthy cats, this makes it hard to use in multiple cat households.
The gold standard
As we gather more information, Y/D may become a staple in our treatment of feline hyperthroidism, but I’m not ready to jump on the bandwagon just yet. In my opinion, if you can manage radioactive iodine treatment, go for it. It’s still the gold standard. If your kitty is currently doing well on oral medication, don’t rock the boat. I’d rather not have my patients be guinea pigs on such a new approach with so little long-term data.
But there may be a very useful place for this product. For cats who are allergic to the medications, for owners who cannot manage any of the traditional therapies, for cases where we are facing euthanasia because a cat is too hard to medicate or money is too tight, or where life circumstances simply make other options impossible, well, anybody can put cat food in a bowl. Talk to your veterinarian. Hills provides specific protocols for starting newly diagnosed cats on Y/D and for transitioning older cases off medication. Regular monitoring is necessary to assess effectiveness and check for recurrence, but in those cases where we have run out of other alternatives, the potential benefits of trying Y/D seem to me to outweigh the risks.