WASHINGTON – Chief Justice William H. Rehnquist’s vow to keep working while battling cancer comes with a large caveat: He’ll remain on the Supreme Court “as long as my health permits.”

The 80-year-old Rehnquist signaled that he is effectively fighting thyroid cancer and his health is strong enough to allow him to preside over the high court for the foreseeable future. How long that will be is an open question.

But it’s clear he is determined – he was back at his desk Friday, one day after being released from a hospital where he spent two nights being treated for a fever.

“That element of uncertainty is always going to be there,” said Craig Bradley, a law professor at Indiana University and former Rehnquist law clerk. “I’m confident barring health problems he does not plan to retire this summer. In another year, who knows?”

Rehnquist, a conservative put on the court by President Nixon in 1972, was diagnosed with cancer last fall and began receiving chemotherapy and radiation. He also had surgery to insert a trachea tube to help him breathe. He has not said whether he has the deadly anaplastic form of the disease or a less serious type.