Thomas, a 49-year-old Briton, was among 100 delegates aboard the plane on their way to an international conference on Aids in Australia.
A total of 298 people were killed when the Malaysian Airlines flight from Amsterdam to Kuala Lumpur was apparently downed by a missile in eastern Ukraine yesterday.
The latest breakdown of the nationalities of those on board the flight is:
- 189 Netherlands
- 44 Malaysia
- 27 Australia
- 12 Indonesia
- 9 UK
- 4 Belgium
- 4 Germany
- 3 Philippines
- 1 Canada
- 1 New Zealand
- four passengers’ nationalities are yet to be verified
Thomas, who had been at the WHO for more than ten years, was previously a reporter for the BBC.
He was responsible for hosting press conferences to promote the WHO’s work to journalists.
Thomas leaves behind his partner, Claudio, and his twin sister, Tracey, the organisation said.
A spokeswoman for the WHO said: "It is with deep sadness that we have to inform you that WHO lost one of our colleagues, Glenn Thomas, on the Malaysian Airlines crash yesterday. His twin sister says he died doing what he loved.
"Glenn will be remembered for his ready laugh and his passion for public health and he will be greatly missed by those who had the opportunity to know him and work with him. Our deepest condolences go to his family, friends and colleagues at this time."
Jane Hill, a former colleague of Thomas at the BBC, said on Twitter: "So shocked and sad to hear my former BBC colleague Glenn Thomas died on #MH17."
Richard Porter, controller of English services for BBC World Service Group, also paid tribute to Thomas and said that his interest in journalism was "only matched by his interest in humanitarian issues" and that former colleagues were "reeling" at news of his death.