Researchers at Washington University School of Medicine in St. Louis found that some regular strength preparations contain as much as 6.5 parts per million (ppm) of fluoride. In the US the maximum level is set a 4 ppm maximum level for drinking water, 2.4 ppm is permitted in bottled water and beverages.
"The tea plant is known to accumulate fluoride from the soil and water. Our study points to the need for further investigation of the fluoride content of teas," says Michael Whyte, professor of medicine, pediatrics and genetics. "We don't know how much variation there is from brand to brand and year to year."
Sales of ice tea are growing rapidly across the world, with the drink gradually moving from Japan and the US, where it is already well established, towards the potentially high-consumption markets of Europe.
Global sales of ready-to-drink ice tea are estimated at $23 billion (€19bn).
A recent report from Datamonitor estimated that French annual iced tea consumption rose to 3.2 litres per person in 2002 - a 65 percent rise on 1997, while the figure in Germany was up by 33 per cent to 6.5 litres.
The US scientists say their discovery stemmed from the diagnostic investigation of a middle-aged woman suffering from spine pain attributed to hyper-dense bones.
Testing for the cause of her symptoms revealed the patient had high levels of fluoride in her urine. She then disclosed a high consumption of iced tea - claiming to drink one to two gallons of double-strength instant tea throughout the day - which led the researchers to test for fluoride content in several brands of instant tea available on supermarket shelves.
Each of the teas was tested as a regular-strength preparation in fluoride-free water, and each contained fluoride, with amounts ranging from 1.0 to 6.5 parts per million.
The study is reported in the January issue of The American Journal of Medicine.
According to Whyte, the findings could aid in the diagnosis and treatment of patients who have achiness in their bones.
Questions surrounding the impact of dietary fluoride on human health still exist.
Some dentists and the World Health Organisation have backed fluoridisation while other experts have said more research is needed on the issue.
Fluoride has been said to help decrease tooth decay and strengthen teeth in young children.
However, others believe the substance can have toxic side-effects and prove harmful to dental health.