The most important androgen in men is testosterone. Androgens are the sex steroids or hormones that produce changes in body shape and sexual characteristics typical of men after puberty. Androgens play a major role in the reproductive and sexual function of the adult male. The equivalent sex steroid produced by women is oestrogen.

Androgen deficiency occurs when reduced levels of testosterone arise from a lack of hormonal drive from the brain or problems with the testes. Replacement of testosterone may be given to such men and this is usually continued for life. About 1 in 200 men under 60 years of age suffer from androgen deficiency. However as men age, testosterone levels begin to fall from the age of 40 years. It is believed that by the age of 65 years, 10% of men will have androgen deficiency and by the age of 70 this figure will have risen to over 20%. Men’s testosterone levels fall much more gradually and over a longer period of time, unlike women, whose oestrogen levels fall rapidly when they go through the menopause.

Men with low testosterone complain of a number of symptoms including easy fatigue, low energy levels, low mood, irritability, poor concentration and reduced libido. Low testosterone may also contribute to erectile problems though androgen deficiency is an uncommon cause of this. As men age, the amount of body fat increases and muscle mass and strength decreases. A fall in testosterone levels is likely to contribute to these conditions. Low testosterone levels are also a risk factor for the development of osteoporosis.