On 14 September 2019, Thomas announced that he is HIV positive, with undetectable status, meaning that he is not infectious. On the following day he competed in the Ironman Wales event in Tenby, finishing 413th out of 2,039, having vowed to "break the stigma" around the illness. After feeling media pressure to do so and for National HIV Testing Week, Thomas also filmed a documentary with Prince Harry, Duke of Sussex and the Terrence Higgins Trust, which aired a few days later on 18 September. Publicising the documentary that morning, he told an interviewer for BBC Radio 5 Live that a journalist had revealed his HIV status to his parents before he had had the opportunity to do so himself. While Thomas declined to name the journalist or the paper they write for, he did say "everybody will know, especially of late", leading the "Press Gazette" to suggest it could be "The Sun", given they had been criticised the previous day for reporting about the family of England cricketer Ben Stokes. A BBC documentary, "Gareth Thomas: HIV and Me", aired in September 2019.