Environmental toxicants are equal-opportunity hazards; mercury, asbestos, pesticides and other compounds can cause health problems in humans and animals alike. For at least a century — since coal miners began using caged canaries to alert them to the presence of toxic gases — we have known that we can put these shared vulnerabilities to practical use. Sick animals can be sentinels, warning of looming threats to human health. For household chemicals, cats and dogs, which tend to spend nearly all their time in the home and happily hoover up whatever detritus falls on the floor, may be particularly useful sentinels. “Our household pets are exposed to many of the same kinds of chemicals that we are,” Birnbaum says. “I think if we see a health problem in our animals, especially one that has arisen very recently — genetics doesn’t change that quickly — I think it’s kind of raising the canary-in-the-coal-mine issue.”

Could hyperthyroid cats be modern-day canaries? We know that flame retardants accumulate in our own bodies; scientists find PBDEs in nearly every person they test, including newborns. “It’s almost 100 percent detection,” says Heather Stapleton, an environmental chemist and exposure scientist at Duke University. The compounds turn up in human blood, breast milk and tissue and can persist for years in fat.

Over the course of decades, human PBDE levels skyrocketed, increasing 100-fold from the 1970s to the early 2000s. (These levels now appear to be declining, most likely as a result of the phasing out of the chemicals.) The rate of human thyroid cancer more than doubled over the same time period. These parallel trends may be more than coincidence: Multiple studies have shown that men and women with high concentrations of PBDEs in their bodies tend to have altered levels of thyroid hormones circulating in their bloodstreams. Last year, researchers reported that thyroid problems were more common among American women with elevated levels of PBDEs in their blood. And at a conference this spring, Stapleton and her colleagues presented findings suggesting that long-term exposure to PBDEs may be a risk factor for papillary thyroid cancer; according to the unpublished data, living in a home with high levels of one type of PBDE in the dust more than doubled the odds of having the disease.

Thyroid hormones also play a crucial role in brain development; a deficiency of these hormones, known as hypothyroidism, may cause neurological abnormalities. If PBDEs cause unusual fluctuations in hormone levels in early life, they may do lasting damage. Scientists have found that those who are exposed to high concentrations of PBDEs in utero or during early childhood score lower on tests of motor skills and cognition. These findings are particularly worrisome given that young children — who are not uncatlike in their behavior, ingesting up to 200 milligrams of dust a day — tend to have higher body burdens of PBDEs than adults. The data are not conclusive, and the underlying mechanisms remain unclear. But further studies of cats could help scientists clarify what’s happening. “I remain convinced that paying more attention to what the animals are trying to tell us is a really good idea,” Rabinowitz says. “There are still many disease outbreaks in animals that remain sort of unexplored or unexplained.”

Rabinowitz, who created the online Canary Database to index papers on animal outbreaks that may be relevant to human health, thinks scientists and clinicians could be more strategic about connecting the dots between species. When he and his colleagues recently investigated the potential health risks of hydraulic fracturing, they discovered that skin problems were common in both the people and the dogs living near gas-extraction sites. “We’re finding that there was really some utility in asking about both people and animals when looking at a new hazard,” Rabinowitz says. He suggests that we consider linking the health records of pets and their owners.

For his part, Peterson remains steadfastly focused on cats, which keep showing up with thyroid hot spots that need to be injected with radiation. He will keep them as comfortable as possible during their stay at the “Hypurrcat Spa,” which is why he has converted the floor-to-ceiling pipe into a scratching post and keeps towel-lined baskets on the cold exam table. At his Bedford Hills clinic, which lacks windows for bird-watching, he has even installed a cage of gerbils in the cats’ line of sight. (“People always say, ‘Are the gerbils upset?’ ” he told me. “I think the gerbils like it, because they get to see new cats.”) Sitting in his Manhattan clinic’s waiting room, where the cats are encouraged to relax on the furniture, he said: “I love the animals. I love the animals more than people, I think.”