Ms Thomas, aged 18, announced her decision to quit the catwalks of Paris, New York and Milan after an intense year of modelling which, she said, opened her eyes to a sleazy backstage subculture of drug use, pressure dieting and eating disorders.
Instead, the British model will forego up to pounds 6,500 a day on the foreign catwalks and return to her family in Norfolk to pick and choose her future work - a decision eased by the fact that she is currently the "face" of Cover Girl cosmetics.
The 5ft 10in teenager's remarks that some designers considered her too fat - at nine stone (57kg) she is technically borderline underweight - has alarmed both the British Dietetics Association (BDA) and the Eating Disorders Association.
Both welcomed her decision yesterday and said her stance would help them to convince other young women that the "glamour" industry is anything but.
Around 70 per cent of female teenagers diet to lose weight. Up to 2 per cent of those go on to develop the two commonest eating disorders, anorexia and bulimia, and the figure can be up to five times higher among dancers, models and athletes, according to the BDA.
Lyndel Costain, a BDA nutrition specialist, said some of the strongest pressures on young women were female body images used in advertising and the fashion industry.
She said: "It is very difficult to give good messages to those I work with in psychiatric units especially when they read that Claudia Schiffer lives on 500 calories a day. But Sarah has given us the ability to give young women real and accurate images based on experience, and it will be enormously useful in breaking down some of the misconceptions and pressures which lead teenagers down this path."
Tim Newton, spokesman for the Eating Disorders Association, said: "It's a very welcome stance that Sarah has adopted.
"It's about time the fashion world got a bit more responsible about the images it is portraying. If they want people who look ill, what kind of message is that sending out to the buying public? It is good that someone is prepared to say that they are not prepared to portray an image of illness as popular fashion."
Ms Thomas's decision to quit follows that of the model Emma Balfour, who said earlier this year that she was going to quit the catwalk because of the prevalence of drug use and anorexia.