Levels of testosterone increase during puberty. The hormone helps boys to develop muscle strength and body hair as well as make sperm.

At some point during the window between ages 30 and 40, men's testosterone levels naturally start to decline.

Estimates vary, but low testosterone is thought to affect 10-40% of older men globally, external - which is thought to translate to about 13 million men in the US.

Those men may experience a low mood, reduced sex drive and loss of muscle mass.

A blood test can check the testosterone level and treatment can be given.

Dr Mohit Khera, who led a US Food and Drug Administration (FDA) expert panel last year on screening and testosterone utilisation in the military, told the BBC that all men over 30 should get screened because it is the top marker of a person's overall current and future health.

"The key here is that many young men have low testosterone levels, which puts them at a disadvantage in terms of muscle mass, energy, and that could be an issue if you are in combat," Khera, a professor of urology at Baylor College of Medicine, said.

But he added: "You have to be careful not to just give someone testosterone unless they do have some kind of symptoms."

The benefits of the hormone replacement therapy, he explained, are increased muscle mass, decreased fat deposition, and decreased risk for depression. Over time, Khera added, it can also help with bone mineral density.

He cautioned, though, that it is not meant for everyone.

"If young men do take testosterone, it can make them infertile," Khera said. "You got to be careful. If someone's in the reproductive years, you need to make sure they understand it's going to make you infertile."

It can also result in a theoretical increased cardiovascular risk, he noted.

Hegseth's announcement comes after US Health Secretary Robert F Kennedy Jr and other administration officials have sought to remove barriers for medical professionals to prescribe men testosterone.

The FDA last month requested the removal of safety and effectiveness language on testosterone replacement therapy product labelling, and proposed loosening prescribing limits for testosterone products.

Additional reporting by Michelle Roberts