John McCain's daughter, Meghan McCain, thanked supporters in a separate post on Twitter.
"We could not have made it this far without you," she wrote. "You've given us strength to carry on."
Politicians reacted to the news with well-wishes for McCain, who has been been away from the Capital since December.
Doctors discovered John McCain’s brain tumor after he underwent surgery in July 2017 to remove a blood clot from above his eye. He was previously diagnosed with melanoma in 1993, 2000 and 2002, and underwent treatment each time. Those cancers did not spread, according to John McCain’s doctors.
>> Related: Photos: Sen. John McCain through the years
Glioblastoma is a particularly aggressive form of cancer and the prognosis is often poor, according to the American Brain Tumor Association. The average survival rate for patients with malignant glioblastoma is around 14 months if the person gets treatment. Around 10 percent of patients with the disease live for five years or longer.