Testosterone is an important sex hormone for both men and women. In men, it is made in the testicles. In women, it is made in small amounts in the ovaries.  

It is important for normal male sexual development. During puberty (in the teen years), testosterone helps boys develop male features like body and facial hair, a deeper voice, and muscle strength. Too much testosterone in men can cause shrinking of the testicles, impotence, an increased risk of heart attack and prostate enlargement with difficulty urinating. Too little can cause fatigue, irritability, depression, erectile dysfunction and reduced muscle mass. 

Combined with oestrogen, testosterone helps with the growth, maintenance, and repair of a woman's reproductive tissues and bone mass. Too much can cause acne, excess hair on the face and body, irregular periods and mood changes. Too little can cause low libido and weight gain.