Irwin Naturals, Testosterone UP, 60 Liquid Soft-GelsBioPerine Enhanced Absorption With Powerful LJ100 Longjack Dietary Supplement Advanced Pro Male Testosterone Support Formula with Physical Peak Performance Boosters Testosterone is a male sex hormone that plays a key role in developing and maintaining masculine characteristics. It is known to promote vitality and wellbeing and is involved in mood, sex drive, muscle mass and muscle strength. As men age, testosterone levels start to decline.