Oklahoma Sen. Tom Coburn announced Thursday he would retire at the end of the current session of Congress, giving up his last two years in office.
His term was scheduled to end in 2016.
Coburn, 65, has been battling a recurrence of prostate cancer, but he said Thursday he wasn't relinquishing his seat early because of his health.
"Carolyn and I have been touched by the encouragement we've received from people across the state regarding my latest battle against cancer," he said in the statement. "But this decision isn't about my health, my prognosis or even my hopes and desires.
"As a citizen, I am now convinced that I can best serve my own children and grandchildren by shifting my focus elsewhere," he added.
Coburn’s resignation is expected to set up a special election for the remainder of his term.
Around the Capitol in the new year, Coburn has sometimes been visibly subdued.
A physician by training, Coburn was elected to the U.S. Senate in 2004. Prior to that, he represented Oklahoma's Second Congressional District in the House from 1995 through 2001. He was first elected to the House in 1994, then re-elected in 1996 and 1998.
Coburn's priorities in the Senate have included reducing wasteful spending and balancing the budget. In his statement, Coburn asserted his pride in serving his constituents.
"Serving as Oklahoma's senator has been, and continues to be, one of the great privileges and blessings of my life," he said.
"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 added. "After all, how else could a country doctor from Muskogee with no political experience make it to Washington?"