The illness, revealed about a week before the presidential election, has prompted speculation that the Supreme Court could have a new member sooner than expected. It's been more than a decade since the last vacancy, a modern era record.
A court spokesman announced the 80-year-old Rehnquist's release from the National Naval Medical Center (search) in suburban Bethesda, Md. The chief justice is expected back on the bench Monday when the court returns from a two-week break.
Details about the seriousness of the cancer have been kept secret. Some colleagues, however, have been upbeat about the prospects for Rehnquist, who is known as a hard-nosed taskmaster.
Justice Clarence Thomas told University of Kansas law students on Thursday that he expected the chief justice back at the court "as unforgiving as ever."
About Rehnquist's cancer, Thomas said: "I feel really badly. He's a good guy."
Even from the hospital, Rehnquist apparently was working this week. He joined with the other court members to refuse to put independent presidential candidate Ralph Nader (search) on the ballot in the battleground state of Ohio and to let Philip Morris USA delay paying $10.5 million in damages to a former smoker while the company contests the amount.
The court spokesman would not say whether Rehnquist participated in a private meeting of the justices on Friday to consider the latest appeals. As chief, Rehnquist normally leads those sessions.
"He is a very resilient, robust, strong-willed man," said Washington attorney Charles Cooper, who clerked for Rehnquist. "He's dedicated to his work and his responsibilities. In keeping with that dedication, he would continue (work) unless completely prevented from it."
The type of thyroid cancer, how advanced it is and Rehnquist's prognosis have not been disclosed. Cancer of the thyroid, a gland in the neck that produces hormones to help regulate the body's use of energy, is generally treatable but can be more aggressive in older people.
He underwent a tracheotomy last Saturday as part of his cancer treatment. Some doctors had said the tracheotomy, in which a tube is inserted into a patient's throat to relieve breathing problems, could indicate Rehnquist had an aggressive or complicated type of cancer.
Rehnquist has been on the court since 1972, chief justice since 1986. He has had other health problems including chronic back pain and a torn leg tendon that required surgery. Besides Rehnquist, three other justices have had cancer. John Paul Stevens had prostate cancer, Sandra Day O'Connor had breast cancer, and Ruth Bader Ginsburg was treated for colon cancer.
Thomas said in Kansas that justices have a tendency to work even when they're sick. "It's a place where people work as if they're paid by the hour," he said.