By Marla Cone - An epidemic of thyroid disease among pet cats could be caused by toxic flame retardants that are widely found in household dust and some pet food, government scientists reported Wednesday.

The often-lethal disease was rare in cats until the 1980s, when it began appearing widely, particularly in California cats. That was at the same time industry started using large volumes of brominated flame retardants in consumer products, including furniture cushions, electronics, mattresses and carpet padding.

Scientists from the Environmental Protection Agency noted a possible connection between hyperthyroidism and flame retardants. The chemicals -- known as polybrominated diphenyl ethers, or PBDEs -- mimic thyroid hormones, so experts have theorized that high exposure in cats could cause overactive thyroids.

Cats that remain indoors and eat fish-flavored canned food were found to be the most highly contaminated.

"We know there is an association between indoor living for cats and hyperthyroidism," said Linda Birnbaum, a senior author of the study and the EPA's director of experimental toxicology. "Our paper does show cats are highly exposed and hyperthyroidism may be due to the high PBDEs. More studies are needed to fully determine this."

A major unanswered question is whether cats are the proverbial canaries in the coal mine, signaling health dangers for their owners. Cats and human beings are the only mammals with a high rate of hyperthyroidism.

So far, no link has been established between human endocrine disorders and exposure to flame retardants. However, "there is growing concern," the scientists wrote.

"It is clear that house cats may be able to serve as sentinels for indoor exposure to PBDEs for humans who share their houses," said Birnbaum, one of the world's leading experts on hormone-altering chemicals.

Brominated flame retardants are ubiquitous outdoors and inside homes. The chemicals have been building up in people and wildlife over the last two decades, particularly in the United States, where human concentrations have doubled every few years.

People in the United States have the highest PBDE levels in humans worldwide, but U.S. cats are even more exposed -- some with levels 100 times greater, according to the study.

Twenty-three cats were tested in the EPA's study, including 11 with hyperthyroidism. The researchers found that the cats with hyperthyroidism had substantially higher levels of a PBDE compound. Symptoms of the disease, which is a leading cause of cat death, include weight loss, rapid heartbeat and irritability.

"Our results demonstrated that cats are being consistently exposed to PBDEs, an endocrine-disrupting environmental contaminant," the research team, led by Janice Dye and Marta Venier of the EPA's National Health and Environmental Effects Research Laboratory in North Carolina, wrote in their study published in the journal Environmental Science & Technology. Because of this exposure, "cats may be at increased risk for developing thyroid hyperplastic changes."

Myrto Petreas, branch chief of environmental chemistry at the California Department of Toxic Substances Control, said that the cat study was small but that it reaffirmed health concerns not only for cats but humans too, "especially children, anyone exposed to high levels."

"PBDEs are in consumer products, so we get exposed while we use the products in homes and during the lifetime of the products. We inhale or ingest dust, mostly from hand-to-mouth transfer," said Petreas, who did not participate in the study.

The risk to cats that eat dry food and live in homes with average contamination is minimal, the study said, while "at the other extreme, maximal PBDE exposure" occurs in cats that eat fish-flavored canned food and live in houses with highly contaminated dust.

Cats that eat canned food containing whitefish, salmon and other seafood are exposed to PBDE levels up to 12 times higher than cats that eat dry food, and five times more than cats that eat poultry or beef canned foods, the study said. The chemicals build up in oceans and other water bodies and magnify in food chains.

However, much of the exposure -- for cats as well as people -- comes from dust, not food.

Cats, while sleeping, often come in direct and prolonged contact with upholstery, carpeting and mattress materials that contain flame retardants. In addition, they often sit on electronic equipment.

"Because of their meticulous grooming behavior, cats would effectively ingest any volatilized PBDEs or PBDE-laden dust that deposited on their fur during such activities," the scientists wrote.

Scientists say toddlers who crawl on floors and put objects in their mouths also can be highly exposed to the chemical-tainted dust, which has been found in most U.S. homes.

In people and cats with the highest levels, Petreas said, "it's explained not by diet, but more contact with contaminated sofas, computers and other consumer products."

Two pervasive PBDEs, used mostly in foam cushions, mattresses and carpet padding, have been banned in the United States since 2004. The ban was spurred by a California law.

However, other brominated flame retardants remain in widespread use.

In June, the California Assembly passed AB 706, written by Assemblyman Mark Leno (D-San Francisco), which would prohibit brominated and chlorinated flame retardants in furniture and bedding. The bill, which now goes to the Senate Appropriations Committee, does not ban their use in electronics.

California has the nation's strictest fire-retardant standards for furnishings, so PBDE exposure is generally higher than elsewhere. The cat epidemic showed up first in California and the Great Lakes region -- the areas with the highest environmental levels of the chemicals.