People could one day extend their lifespan by up to 30 years by taking a pill, a scientist has claimed.
Scientists are searching for a pill which will extend healthy life
Professor John Speakman, from Aberdeen University, said the hormone thyroxine could boost metabolism and so lifespan.
He said tests on mice suggested, if the right dose could be identified, humans who would otherwise have died aged 70 could live to 100.
But a leading hormone specialist warned too much thyroxine could cause potentially fatal health problems.
Studies carried out by the UK team showed the mice with the highest metabolic rate lived around 25% longer than those with the lowest.
Professor Speakman said this would translate to a difference of around 30 years in humans.
When mice were given thyroxine, they had increased metabolic rates and lived longer, compared with animals which were not given the hormone.
Thyroxine is already given to people who do not produce enough of the hormone naturally, so that they have a healthy metabolic rate.
But people with too much thyroxine in their bodies also have to take medication to bring their level back to normal.
Thyroxine boosts the body's metabolic rate which has a beneficial effect on cell biology, setting off a process which reduces the production of damaging free radicals.
Professor Speakman, who has been awarded a £450,000 grant by the Biotechnology and Biological Sciences Council, said: "We know thyroxine affects your metabolic rate. The key is getting the right dose."
He will now carry out research with mice to determine the most effective dose of the drug.
Professor Speakman said it might not be possible to find a level that did not have detrimental effects on human health.
But he said there were other molecules which could have the same effect on uncoupling proteins.
'Not true for humans'
Professor Speakman said: "The end point of this research is the hope we'll be able to give people extra healthy years. We don't want to extend their stay in a nursing home."
His work is set to be published in the Journal of Experimental Biology.
However, a leading specialist in human hormone disorders said the findings would "not be true for humans".
Dr Pierre Bouloux, an endocrinologist at the Royal Free Hospital in London, said: "This is an example of research being extrapolated on the basis that a mouse represents the best model for a man. It doesn't.
"Mice have a different metabolism to humans."
He added: "Having an over-active thyroid gland puts you at a three-fold risk of potentially fatal heart disorders and a three to four-fold risk of osteoporosis.
"An over-active thyroid causes considerable morbidity in the ageing population."
And he warned people who had even a slightly higher level of thyroxine were at risk of ill health.