The question “what is gender” is not a simple one to answer—at least it is not for the officials of the International Olympic Committee (IOC). Athletes competing at the Olympics must be separated into men’s and women’s events. The IOC has to have a generally accepted method to sort the women from the men since they are not allowed to compete in the same events.
Gender Issues Big Headache for IOC

New guidelines for the IOC allow athletes who previously identified as female to compete in the male category without restriction. The reason is because male athletes would not gain an advantage from their previous gender.
On the other hand those who transition from male to female must meet several requirements. According to FiveThirtyEight writer Christie Aschwanden, the athlete must declare a female identity, must also document that her total serum testosterone levels have remained below a certain limit for a minimum of 12 months before competing, and these levels must remain under the threshold as long as she is competing.
Caitlyn Jenner, who used to be the swimmer Bruce Jenner, believes the IOC decision is a “huge step forward for everybody in the [transgender] community.”
William Briner, M.D., a sports medicine physician at the Hospital for Special Surgery in Uniondale, New York, notes that people who go through male puberty are taller, have bigger bones and develop greater muscle mass than those who go through female puberty. Men also have more red blood cells than women, and their hearts and lungs are bigger too. Some of these advantages are irreversible and they give men a distinct advantage over women.
This is the reason that sports performers are segregated by gender. That still leaves officials with the task of figuring out how to separate the women from the men.
The basic issue for the IOC is one of fairness. What constitutes fairness and how do officials insure it? Also to be determined is that balance between protecting the integrity of women’s sports while also respecting the rights of transgender athletes, who already face discrimination and marginalization when they participate at a high level in sports.

Discussion
This is a fascinating development. In my practice we've seen similar outcomes with the revised protocol. The key differentiator seems to be patient selection criteria. Has anyone else noticed the correlation with BMI thresholds?
Great point. I'd push back slightly on the conclusion, the sample size in the cited study is too small to draw population-level inferences. That said, the directional signal is compelling and worth a larger RCT.
We implemented a similar approach last year. Early results are promising but we're still gathering 12-month follow-up data. Happy to share our protocol if anyone is interested.
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