Defence Secretary Pete Hegseth has announced the US military will start testing the testosterone levels of service members in what he described as an initiative "about restoring and optimising" their capabilities.

In a video posted on X, captioned "High-T Department", Hegseth said he is authorising the screening program for troops to ensure "you have the right testosterone levels to operate at your absolute best".

Those 30 and older will be screened annually, while those who are younger can request testing. Troops with low levels will be offered voluntary testosterone replacement therapy.

It comes as others in the Trump administration have pushed for expanding access to such therapies.

"We owe our warriors the absolute best medical care in the world, and this program delivers on that obligation," Hegseth said in the video.

"Taking care of your long-term health means ensuring you remain strong, resilient and capable — not just for your next deployment, but for the rest of your life, so you can thrive long after you take off the uniform," he added.

Taking testosterone for non-medical reasons, like artificial muscle enhancement without a doctor's prescription, is strictly prohibited in the military.

Hegseth said in Wednesday's video that the new program is "not about artificial enhancement".

Pentagon spokesperson Sean Parnell said in a statement on Wednesday that the mandatory screening for testosterone deficiency will be effective immediately for all active-duty and reserve-component personnel aged 30 and older.

"The protocol will enable the Department to establish a comprehensive baseline and offer targeted testosterone therapy, ensuring that it sustains a healthy, capable, and decisively dominant fighting force," he said.

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 on Wednesday 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 said, "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 health secretary has floated it as a solution to a national "fertility crisis".

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.