Sen. Tom Coburn, an outspoken Oklahoma Republican who on occasion bucked his party leaders, announced late Thursday he will step down from office when the 113th Congress ends in early January 2015 — two years before his term expires.

Coburn, 65, who has been battling cancer, said he's not quitting for health reasons but rather to uphold a promise he made when first elected to the Senate that he wouldn't serve more than two six-year terms.

"This decision isn’t about my health, my prognosis or even my hopes and desires," said Coburn in prepared statement. "Our founders saw public service and politics as a calling rather than a career. That’s how I saw it when I first ran for office in 1994, and that’s how I still see it today.

"I believe it’s important to live under the laws I helped write, and even those I fought hard to block. "

Coburn was elected to the House in 1994 as part of former GOP Speaker Newt Gingrich's "Republican Revolution." He promised voters at the time he would serve no more than three consecutive terms, a pledge he honored when he didn't run for re-election in 2000.

But four years later, he successfully ran for Senate. He won re-election in 2010 with 71 percent of the vote.

Coburn, a physician and staunch fiscal and social conservative, has been an outspoken critic of "earmarks" — pet spending projects that members of Congress slip into legislation.

He was known on Capitol Hill for his annual "Wastebook," a report that highlighted wasteful spending with the federal government.

"As dysfunctional as Washington is these days, change is still possible when ‘We the People’ get engaged, run for office themselves or make their voices heard," Coburn said. "After all, how else could a country doctor from Muskogee with no political experience make it to Washington?"

Coburn's office announced in November the senator was seeking treatment for a recurrence of prostate cancer that first was diagnosed in 2011. He also has battled colon cancer and melanoma in the past.

"As a citizen, I am now convinced that I can best serve my own children and grandchildren by shifting my focus elsewhere," he said. "In the meantime, I look forward to finishing this year strong."