Doctors often recommend that men with erectile dysfunction have their testosterone measured from 8 a.m. to 11 a.m., because levels are usually highest then and vary over a 24-hour period. Researchers retrospectively studied records of 2,569 men ages 26 to 84 who were evaluated at an erectile dysfunction clinic from 1985 to 2004. They sorted them into age groups, starting with 26- to 39-year-olds, and then in five-year groups up to age 84.

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