This undated photo provided by Eli Lilly & Co. shows the drug Axiron, an underarm gel used for boosting levels of testosterone. Men's bodies convert some of the male sex hormone into estrogen, and levels of both decline with age. Until now, there was no way to tell which hormone was responsible for complaints of diminished libido, strength and energy. Millions of men have been prescribed testosterone gel, patches or shots for these problems, but it's not known how much they need or whether this hormone replacement therapy is good, bad or neutral, according to Dr. Joel Finkelstein of Massachusetts General Hospital. He led a federally funded study, published in the Thursday, Sept. 12, 2013 New England Journal of Medicine.