Scientists say testosterone replacement therapy, known as TRT, could provide 'rapid and sustained' improvements in men's sexual performance, energy, and mood. 

Testosterone is the male sex hormone that controls everything from sexual performance to muscle growth, but millions of men are living with sub-optimal levels. 

In a new study, published in The World Journal of Men's Health, scientists from Harvard University analysed how over 9,500 men with low testosterone responded to TRT.  

They found that supplementing testosterone led to significant improvements across all quality of life measures assessed, including: energy levels, strength and endurance, life enjoyment, happiness, erection strength, work performance, and sporting ability.

Improvements were noticeable as early as two months after starting treatment and were similar for all men, regardless of baseline testosterone levels. 

Dr Jeff Foster, director of men's health at Voy, the largest digital provider of TRT, said: 'Our results give one of the clearest pictures to date of how TRT performs across thousands of men and a wide range of treatment approaches.

'When testosterone levels fall too low men can experience fatigue, low mood, weight gain, reduced libido, erectile dysfunction, and poor concentration, amongst other symptoms.'

According to the NHS, some men will also develop depression when they reach their late 40s and testosterone levels start to fall. 

One of the biggest factors driving low testosterone is rising levels of obesity

Other symptoms include fat redistribution, such as developing a large belly or man boobs, difficulty sleeping and loss of muscle mass.  

Dr Foster added that the results 'reinforce the importance' of treating patients'  individual circumstances, and not just solely relying on blood tests. 

He said: 'Treatment guidelines should take into account a man's age and symptom severity alongside their specific testosterone profile to ensure the best possible results and minimise misuse.' 

According to the latest research, men experience around a 1 per cent drop in testosterone levels a year from around the age of 30 to 40-years-old. 

And whilst this is unlikely to cause any problems in itself, low levels can increase the risk of a number of serious health problems including heart disease, osteoporosis, infertility and is linked to a 40 per cent increased risk of early death. 

Health professionals have long warned that many men remain undiagnosed due to a lack of knowledge and willing to discuss hormone issues. 

Part of the reason it goes undetected is because the symptoms can mimic other problems.

The most common relate to sexual dysfunction – unsurprising given the leading role testosterone plays in the male libido. This might include problems getting or maintaining an erection, producing less semen, or a reduced sex drive.