McCain's daughter, Meghan McCain, also posted a tweet: "My family is deeply appreciative of all the love and generosity you have shown us during this past year. Thank you for all your continued support and prayers. We could not have made it this far without you - you've given us strength to carry on," she wrote.
Read the family's statement:
Last summer, Senator John McCain shared with Americans the news our family already knew: he had been diagnosed with an aggressive glioblastoma, and the prognosis was serious. In the year since, John has surpassed expectations for his survival. But the progress of disease and the inexorable advance of age render their verdict. With his usual strength of will, he has now chosen to discontinue medical treatment. Our family is immensely grateful for the support and kindness of all his caregivers over the last year, and for the continuing outpouring of concern and affection from John's many friends and associates, and the many thousands of people who are keeping him in their prayers. God bless and thank you all.
The news of McCain's decision, while not entirely unexpected, was met with deep sadness from his Senate colleagues on Friday.
"Becoming John McCain's friend has been one of the great blessings of my life," former Sen. Joe Lieberman, one of McCain's closest friends, told CNBC in a statement. "Today I am praying for him and his family."
Few legislators in American history have served for more years, or in more capacities than McCain has during a Senate career that began in 1987 and lasted more than three decades. At various points during his tenure, McCain has chaired the Armed Services Committee, the Indian Affairs Committee and the Senate Commerce Committee.
McCain first garnered national attention during the Vietnam War, when, as the son of an admiral in the U.S. Navy, he spent more than five years as a prisoner of war, an ordeal that left him unable to lift his arms above his head.