Semenya has a condition known as hyperandrogenism, which is characterised by higher than usual levels of testosterone, a hormone that increases muscle mass and strength and the body's ability to use oxygen.
Gender tests on Semenya reportedly showed the runner had internal testes, the male sexual organs which produce testosterone, and her levels of the hormone were three times that of a "normal" female. If you want to see I'm a woman, I will show you my vagina. "They thought I had a dick, probably," Semenya told HBO.
Shortly after Semenya had announced herself as a global talent by taking the 800m world title in Berlin by a huge margin, the sport's world governing body – then the International Association of Athletics Federations (IAAF) - said she would undergo gender verification tests.
The 31-year-old South African middle-distance runner, who also earned 800m Olympic gold medals at London 2012 and Rio 2016 before being ruled ineligible to carry on racing over that distance due to her naturally elevated testosterone levels, revealed the incident during an interview with HBO's Real Sports.
Caster Semenya offered to show her vagina to athletics officials in 2009 when she won her first world 800 metres title aged 18 to prove she was female.