That said, there has been relatively little scientific study of elite transgender athletes. And while testosterone’s role in physical strength and stamina is robust, studies have not been able to gauge its precise impact on performance.

Last year, World Athletics, the world governing body for track and field, which has imposed strict restrictions on runners who compete in some women’s events, corrected its own research, acknowledging that it could not find a causal relationship between high testosterone levels and enhanced athletic performance among elite female athletes.

It is not clear whether Thomas’s performance prompted FINA to pass such a strict rule about participation.

Thomas, who competed for the University of Pennsylvania, was lauded by supporters as a brave and courageous athlete this year, but her performance also sparked a backlash. Members of her own team complained about her participation, and a group of swimmers at Princeton went to the commissioner of the Ivy League to lobby against her.

Several states have passed laws barring transgender women from competitions. Some states, including Texas, have moved to outlaw medical intervention for transitioning for younger children, which would disqualify them from meeting the FINA regulations.

The question now is whether the swimming federation’s strict ruling will have a trickle-down effect. Harper, who supports sport-specific restrictions on testosterone levels for transgender women at the international level, said she was concerned that even local organizations would feel justified in barring transgender athletes. She citied the recent case of a 60-year-old transgender woman who was prevented from participating in a lawn bowling competition.

“At the international level, there is some logic to do this,” she said of the restrictions. The danger, she said, is that the people making these decisions at the recreational level “will look at FINA and put these regulations on middle school kids.”

Billy Witz, Jeré Longman, Azeen Ghorayshi and Livia Albeck-Ripka contributed reporting.