Ms Thirkettle, 48, who had been suffering from cancer, covered more than 1,500 stories for ITN after joining as a reporter in 1974.
She reported on the famine in Ethiopia, Richard Branson's trans-oceanic balloon expeditions, highlighted the artistic talent of autistic boy Stephen Wilshire and was a member of the ITN team which won a Royal Television Society award for the coverage of Labour leader John Smith's death.
Ms Thirkettle began her career in 1965 as a trainee researcher with Associated Rediffusion. She joined the Daily Mail, and, in 1969, went to the Sunday Times as property correspondent and business writer.
From 1970 she reported for BBC radio and British Forces Broadcasting before becoming a founder member of London Broadcasting Company (LBC) in 1973. She joined ITN the following year and rapidly became a household face and name.
When not reporting, Ms Thirkettle wrote short stories and studied natural history, politics and foreign affairs. She was also an occasional presenter of music programmes onClassic FM radio.
Colleagues and friends paid tribute to Ms Thirkettle's "sheer professionalism and skill": "Joan's death is a great loss," ITN's chief executive Stewart Purvis said. "For more than 20 years she was part of the fabric of ITN, part of its history and part of its family. She covered a wide range of stories with determination, professionalism and great integrity."
Fellow ITN newsreader Julia Somerville said: "Among all her other great qualities she was also extremely kind-hearted."
Richard Branson, Virgin Atlantic chairman, described her as "a great ambassador for ITN".
Ms Thirkettle was divorced and lived in London with her two children, Daisy, 18, and Michael, 15.