Testosterone is a sex hormone present in all humans, at higher typical levels in men. It supports muscle, bone, red cells, voice changes in puberty, and a share of libido. Levels vary across a day and across a life.

Low levels can flatten desire and erections and energy. High supplemental levels bring acne, sleep apnea risk, fertility loss, and a temptation to treat a mood problem as a hormone shortage. Blood tests belong before anyone starts a gel because an ad promised their younger self back.

Gender-affirming testosterone for trans men and some nonbinary people is a separate, monitored medical path. Sports and clinic guidelines around that path are fought in public. The chemistry remains a tool that changes bodies and needs follow-up.

Sleep and resistance training still influence how a body uses what it has.