US Defense Secretary Pete Hegseth has announced a new testosterone screening program that he says will help troops operate at their "absolute best".Troops under 30 will be able to volunteer for testosterone testing and service members above that age will undergo compulsory screening as part of their annual medical review.Mr Hegseth said in a social media video that the program would also give troops the opportunity to undergo testosterone replacement therapy.It was unclear if he was only talking about testing male personnel.When asked what conditions the policy would address, the Pentagon referred to Mr Hegseth's remarks in the video that mentioned keeping troops "strong, resilient and capable" and said the rigours of the modern battlefield demanded "maximum psychological and mental readiness".Over the last several years, special operations troops — specifically Navy SEALs — have come under scrutiny for their use of testosterone and similar substances to enhance performance.The 2022 death of a SEAL recruit revealed widespread testosterone use among elite US soldiers. (Supplied: Az Cosgrove)The death of a SEAL recruit during training in 2022 led to the discovery of substances in his possession, including testosterone, and revealed far more rampant drug use among the elite program than was previously acknowledged.A year after the recruit's death, the Navy said it would begin a drug-testing program to screen for "any hormonal substance, chemically or pharmacologically related to testosterone, that promotes muscle growth".Mr Hegseth said his new initiative was "not about artificial enhancement".The Pentagon did not respond to questions about what research or academic studies underpinned the move.It also did not say if female troops would be able to be evaluated for estrogen-based therapy as they entered perimenopause.Testosterone levels in men decline naturally with age and have long been linked to issues like erectile dysfunction, low libido, mood changes and weight gain.But experts have debated for years how to diagnose those problems and whether they should be treated with hormone replacement.Mr Hegseth's announcement comes as Health Secretary Robert F Kennedy Jr and other Trump administration officials move to make it easier for doctors to prescribe testosterone.Last month the Food and Drug Administration proposed that prescription limits on testosterone gels, pills, patches and injections be eased.The current FDA label specifies that the medications are only for men with hypogonadism, a medical condition that causes drastically low testosterone.But many influencers and proponents of Mr Kennedy's "Make America Healthy Again" movement tout testosterone as a way to look younger, build muscle and stay mentally sharp, though those uses are not accepted by most medical experts.Recent studies have bolstered the case for testosterone's benefits for certain conditions while allaying worries about its safety, particularly concerns about heart safety.Last year the FDA removed a warning about possible risks of heart attack and stroke from the drugs.Senator Tammy Duckworth, a Democrat and Iraq War veteran, said the announcement sounded "like gender-affirming care", a dig at Mr Hegseth's stance against transgender troops.Democrat Chrissy Houlahan, an Air Force veteran, said it "proves that Secretary Hegseth takes direction from the far corners of the manosphere".Both called on Mr Hegseth to make hormone testing available for men and women.Pete Hesgeth says the program will allow troops to undergo testosterone replacement therapy. (NurPhoto via AFP: Allison Bailey)"Let's extend hormone screenings for all of our brave service members to help us identify fertility issues early — since studies show that both women and men in our military disproportionately face higher rates of infertility than the general population," Ms Duckworth, a Senate Armed Services Committee member, said in a statement.Mr Hegseth has previously said he did not believe women should hold combat roles and that those positions should be based on the "highest male standard".He has blocked some military promotions for women and fired female leaders since becoming Pentagon leader.In April Mr Hegseth announced the repeal of the military's long-standing flu vaccine mandate, citing "medical autonomy" and religious freedom.A flu outbreak at a US Air Force boot camp affected hundreds of people in June.AP/ABC