Sen. John McCain expressed on Thursday his gratitude for the support that he has received since he was diagnosed with an aggressive brain tumor – along with some "unfortunate" news for some of his colleagues.

“I greatly appreciate the outpouring of support – unfortunately for my sparring partners in Congress, I’ll be back soon, so stand-by!” McCain, R-Ariz., tweeted Thursday morning.

McCain, 80, was diagnosed with the tumor after he underwent an earlier craniotomy to remove a blood clot above his left eye, according to the Mayo Clinic Hospital in Phoenix.

McCain and his family are reportedly reviewing treatment options.

His office has said the senator is “in good spirits as he continues to recover at home with his family in Arizona.”