"...if there is no deficit to the hormone". It sounds like if someone has lower-than-normal testosterone then it increases sex drive. But if someone is already at normal levels of testosterone takes more, it doesn't significantly change sex drive.

No, the headline/study match-up is even dumber than that I'm afraid.

The study (which looks fine, the article is trash) is looking at if a given man's sexual desire fluctuates as their testosterone does, day to day[0], in men with healthy testosterone levels.

https://royalsocietypublishing.org/doi/10.1098/rspb.2024.150...