It's normal to have a lower sex drive and fewer spontaneous erections as you get older, but little or no libido can be a sign that you have low testosterone. Because the symptoms of low testosterone can be vague and because men don't always mention their symptoms to their doctors, the actual number of men with low testosterone levels is unclear.

A decline in testosterone levels is normal with ageing. Levels gently fall from age 20 onwards and at 75, may be half that of a 20 year old. However, testosterone levels that are considerably lower than what's normal for your age is not a normal part of ageing. It can affect your quality of life and health and it can be remedied.

Low testosterone can alter some typical masculine features in men. Painful breasts or breast enlargement may suggest low testosterone in men. Other signs are losing body hair and having to shave less frequently. Also, your testicles may be smaller when testosterone is low and it may be difficult to get or keep an erection. Hot flushes are most often seen in men who have very low testosterone.

Testosterone doesn't just relate to sex. It helps build bone and low testosterone can lead to frail bones and osteoporosis. Testosterone may also help keep muscles strong. It helps produce red blood cells, boosts mood and helps thinking. Low testosterone may cause anaemia, depression and trouble concentrating.

Many men with low testosterone don't have symptoms. Only a blood test can tell what your testosterone levels are. However, the British Society for Sexual Medicine says diagnosis should be based upon symptoms such as weight gain, poor morning erection, low sexual desire and erectile dysfunction, combined with a blood test taken in the morning on more than one occasion. Patients with serum total testosterone levels below 8nmol/l (230ng/dl) will usually benefit from testosterone treatment.

From puberty, when a boy starts to get a beard and pubic hair, testosterone affects hair growth in men. However, it doesn't affect growth on all parts of the body in the same way. Low testosterone can cause you to lose body or facial hair, but it doesn't cause male pattern baldness. Genetics have more to do with that.

If you are healthy and have no symptoms of low testosterone, there is no need to be tested for it. However, many men with type 2 diabetes have low testosterone. If you have type 2 diabetes, you should talk to your GP about being tested. Men who are infertile, have chronic obstructive pulmonary disease (COPD), osteoporosis or a fracture from an accident should also consider being tested.

Low testosterone can be a cause of erectile dysfunction (ED) - the inability to get or keep an erection - and testosterone therapy may help treat it. However, in some cases, testosterone treatment does not improve ED symptoms because there are other causes. For example, diseases of the nerves and blood vessels can also affect the penis and cause ED. If you have ED, seek medical advice.

Around the age of 40, testosterone levels start to fall about 1% a year.  This is a normal decline that occurs as men age. Men whose testosterone levels are below normal may or may not have symptoms of low testosterone. When they do, your doctor may recommend treatment with testosterone therapy.

Research has shown that nearly 40% of obese men over age 45 have a low testosterone blood level. About 50% of obese men who also have diabetes have a low testosterone level. Men with metabolic syndrome are also at a high risk.

Testosterone replacement therapy can be used to increase testosterone levels but you should discuss with your doctor whether it is right for you. Injections, usually given every few weeks, are one of the oldest ways to increase testosterone. Testosterone can also be absorbed through the skin, using patches or gels. Another option is a mouth patch. The British Society for Sexual Medicine guidelines say, once patients are on therapy, their levels should be monitored and the aim should be a total testeosterone level of at least 15nmol/l to ensure improvement in symptoms.

Anabolic steroids, often used by athletes and weightlifters, mimic the body's natural testosterone in bulking up muscles. However, using them can actually lower testosterone levels. Excess anabolic steroids may also increase aggression in men, cause severe acne or trembling, shrink testicles, reduce sperm count and cause baldness.

Finasertide, used to treat male pattern baldness, can increase testosterone blood levels. Other medicines can lower testosterone. These include corticosteroids such as prednisolone, used for inflammatory conditions, and long-acting narcotics, like oxycodone and morphine. Medicines used to treat advanced prostate cancer can also lower testosterone levels.

Testosterone replacement therapy may make some medical conditions worse, such as sleep apnoea, which causes pauses in breathing during sleep. It is not recommended for men with severe sleep apnoea, severe benign prostatic hypertrophy or lower urinary tract infections. Other illnesses that don't mix with testosterone therapy include prostate cancer, male breast cancer and congestive heart failure that is not well managed.

Alcohol is directly toxic to the testicles, where testosterone is produced, and it seems to affect the release of other hormones related to men's sexual function and fertility. Shrunken testicles are a common sign of low testosterone in alcoholic men with liver disease, as well as lower libido and reduced erectile function. Enlarged breasts are common in heavy drinkers because alcohol may help convert testosterone into the female hormone oestrogen.

Testosterone therapy does more than give your libido a lift. Studies show that it may improve thinking and energy and that it does improve mood. It may also increase muscle strength and mass, as well as help treat osteoporosis. It may even make you more alert, but it won't directly help you play the piano.

Up to nine out of 10 men who have symptoms of low testosterone may not get treatment. They may attribute their symptoms to other conditions or think they're a normal part of ageing. If you have symptoms and believe they are having an impact on your quality of life and well-being, seek medical advice.