It’s not just the human population that is living longer than ever before; with better medical care, more and more pets are living to old age, too. The downside, of course, is an increasing incidence of age-related diseases.

One of these is hyperthyroidism, a condition seen commonly in older cats. Typically the result of a non-cancerous tumor leading to an overproduction of thyroid hormone, the disease can have severe consequences if left untreated—even death. The School of Veterinary Medicine’s Ryan Hospital, however, is one of the few veterinary centers in the region to offer a treatment that can cure the disease: a single shot of radioactive iodine, I-131.

Cats with hyperthyroidism exhibit symptoms that are similar to those of humans with the disease: weight loss, increased appetite, excessive drinking, agitation, and restlessness.

“Their owners may find that their cats are waking them up meowing multiple times a night,” says Ariel Mosenco, clinical associate professor of medicine at Penn Vet, who leads the I-131 therapy program at Ryan Hospital.

Medication is available to treat hyperthyroidism, but owners must give their cats the pills daily, sometimes multiple times a day, for the rest of the animals’ lives. Cats taking the medication require periodic blood tests to monitor their condition, and the drugs can also have serious side effects in a small number of animals.

I-131 therapy, in contrast, has almost no side effects and has a cure rate around 95 percent. The treatment entails a one-time injection of radiation-emitting iodine, which is taken up preferentially by the thyroid gland. The I-131 destroys thyroid cells and thus decreases the amount of thyroid hormone the gland releases. Cats usually receive the injection on Monday and are ready to go home by Thursday or Friday of the same week.

Ryan Hospital is specially equipped to administer the radiation treatment. Penn’s Office of Environmental Health and Radiation Safety ensures that clinicians have the proper training in how to handle radioactive materials and dispose of waste. Mosenco and his colleagues wear badges that track their radiation exposure, and monitor the cats with a Geiger counter to determine when they are safe to release to their owners. After receiving the injection of I-131, the cats stay in a separate, quiet room of the hospital in individual cages, regularly checked by the nursing staff.

Though a cat receives a radiation dose significantly lower than the amount a human patient undergoing radiation therapy receives, owners must still take precautions for two weeks after their pet comes home, keeping it away from children and other pets, for example.

Mosenco notes that not every cat is a good candidate for the treatment. Cats with renal disease might do better on medical treatment. In some cases, cats may swing from producing too much thyroid hormone to too little, in which case they may require an oral supplement of the hormone—but that situation is uncommon.

In all, Mosenco is seeing an increasing demand for the treatment, which he considers the “gold standard of care” for hyperthyroidism.

“Studies show that cats that get the radioactive iodine treatment live longer than cats that get only medication,” he says. “As cats live longer, it’s a way to give them a good quality of life.”