WASHINGTON – When she was diagnosed with hypothyroidism in 2004, Phoenix resident Jody Clute had no idea it would lead her, and her city, to revisit one of the biggest public-health debates of the 20th century — water fluoridation.

After her diagnosis, Clute read “every book she could get her hands on” about the thyroid, and they all said to stay away from fluoride.

But that’s nearly impossible with fluoride in the drinking water, even in some bottled waters. So Clute started asking questions, and started asking the city to do the same.

The City Council has agreed to take a look, making Phoenix the largest city to reconsider fluoridation. On Sept. 11, a council subcommittee will hear testimony on the fluoridation of the city’s water supply and vote whether to send the issue to the full council for review.

“No one had even thought about it or even thought to think about it,” Clute said of the council. “Fluoride in water was like air.”

And just about as harmful, supporters of fluoridation say.

“It’s like saying you treat some people with oxygen because they have lung disease — but having oxygen in the air is wrong,” said Steven Schonfeld, a spokesman for the American Dental Association. “Their argument is just bogus.”

Arizona Dental Association Director Kevin Earle said challenges to fluoridation are based on “junk science.”

“It’s like adding vitamin C to your milk, a supplement to your bread. They are not harmful supplements,” he said.

Clute understands the stereotype of the anti-fluoride activist, the person who sees a threat in what the Centers for Disease Control and Prevention identified as one of the top 10 public-health achievements of the 20th century.

“I’m not a conspiracy theorist, and I’m not a hippie,” Clute said.

Forty-two of the 50 largest U.S. cities are fully fluoridated, a process that was popularized in the 1950s as a cheap, effective way to prevent tooth decay. Phoenix began fluoridating its water 22 years ago, and nearly 57 percent of Arizona residents got fluoridated water in 2010, according to the CDC.

Cities that fluoridate their water reduce cavities by 20 to 40 percent, according to the national association.

Phoenix spends $582,000 a year on water fluoridation, according to a spokesman for Phoenix’s Water Service Department, which averages out to just under 40 cents per resident per year.

The Environmental Protection Agency limits the presence of fluoride in drinking-water systems to 4 parts per million.

But opponents argue that while officials can control the amount of fluoride in the water, they cannot control the amount of water that people consume.

“You’re giving every human the same medicine, and you have no control over how much they consume,” Clute said. “It’s in everything, everything with water.”

Other opponents say that any amount of fluoride is still too high.