According to these new rules , athletes have seven days to lower their testosterone levels below 5 nmol/L, the restricted limit as decided by the IAAF in 2018. If their T levels are higher, they can be disqualified from competitions. To put that in perspective, 5 nmol/L is three times greater than the upper limit of normal testosterone in women, Clare Flannery , MD, an endocrinologist at Yale School of Medicine, told Refinery29 . Considering testosterone can aid in muscle growth and oxygen-carrying capacity, the rationale is that athletes — specifically track athletes — could have an upper hand during competition. However, some say that testosterone isn't the most useful metric to level the playing field, because there's not enough evidence that "differences of sexual development" would benefit athletes in all other events.