Male hormones are called androgens. The most important is the hormone, testosterone. The testicles produce more than 95% of the body’s testosterone.

Testosterone affects men's bodies in a number of ways. It:

• alters the balance between fat and muscle in the body. Testosterone reduces the amount of body fat and increases the amount of muscle tissue. If a man has very low levels of testosterone he will tend to have more fat and less muscle than men who have normal levels of the hormone.
• increases the density and strength of the bones. A man with low levels of testosterone is at greater risk of the bones becoming thin and brittle (osteoporosis). This can lead to bone pain and fractures.
• stimulates the production of sperm and semen.
• increases the growth of facial and body hair.
• It tends to reduce the levels of these fats (lipids) in the blood. This may help protect against heart disease and heart attacks.
• may affect behaviour. Increased levels of testosterone can lead to a tendency to be more aggressive.
• affects sex drive. But this varies from man to man. Some men with low levels of testosterone can still have normal erections, be sexually excited and have a normal sex life but others lose interest in sex (loss of libido) or have problems with getting erections or becoming sexually excited.
• may affect mood. Some men with low levels feel low, or depressed and tired but some men don't notice any effect.