I read that having normal testosterone levels is not only important for men, it plays a key role for women as well. For women, these sex hormones are thought to have important effects on Ovarian function, i.e., for the ovaries to work normally; maintain or increase Bone strength and may in a good way, regulate sexual behavior.
In men, testosterone is the major sex hormone and plays a number of important roles, including
1. Development of the penis and testes
2. Deepening of the voice during puberty
3. Appearance of facial and pubic hair starting at puberty; later in life, it may play a role in balding
4. Muscle size and strength
5. Bone growth and strength
6. Sex drive (libido)
7. Sperm production
Testosterone may also help maintain normal mood - for both men and women. In adolescent boys, if they have too little or a low level of testosterone, they may not experience normal masculinization. For instance, the genitals may not enlarge, facial and body hair may be scant and the voice may not deepen normally.
Source: MSN Health