The BBC has more on recent research into metabolism, healthy life extension and the hormone thyroxine. While the potential of an improved metabolism to lead to a somewhat extended healthy life span is demonstrated by calorie restriction, tinkering with thyroxine on an ad hoc basis - as for any hormone - seems to be a potentially dangerous proposition. "We know thyroxine affects your metabolic rate. The key is getting the right dose. ... 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. ... he warned people who had even a slightly higher level of thyroxine were at risk of ill health."