A new study found that men who live the longest are those who have medium testosterone levels. High or low testosterone levels are linked to reduced mortality. Testosterone is a key male sex hormone involved in maintaining sex drive sperm production and bone health. Physicians have long known that low testosterone levels can signal health problems but the new study found men may not fare better when levels of the hormone rise too high. 'Older men who had testosterone in the middle range survived longer than their counterparts who had either low or high levels of the hormone' the study's