The wife of former President George H.W. Bush has been hospitalized since Monday at Methodist Hospital in Houston. There was no timetable for her release, but "we're hopeful that will take place sooner rather than later," family spokesman Jim McGrath said.
McGrath said physicians merely may be exercising extra caution by keeping her at the hospital.
"She is 88," he said. "She is a national treasure."
She and her husband, the 41st president, live in Houston. They are the longest-married presidential first couple and will celebrate their 69th wedding anniversary on Jan. 6.
The former first lady had a reputation for bluntness when her husband was president. Her son, George W. Bush, was the 43rd president.
George H.W. Bush tweeted, "Barbara thanks @BarackObama & @billclinton for their get-well wishes and is heeding their advice. Doesnt happen w every President she knows!"
The Bush family matriarch had heart surgery in March 2009 for a severe narrowing of the main heart valve. She also was hospitalized in November 2008, when she underwent surgery for a perforated ulcer. In 2010, she was admitted to the hospital after having a mild relapse of Graves disease, a thyroid condition for which she was treated in 1989.
But health concerns had been more common of late with her 89-year-old husband, the nation's oldest living former president. George H.W. Bush was released in January 2013 after he spent nearly two months at Houston Methodist Hospital, being treated for a bronchitis-related cough and other health issues.
The former first lady and her husband still make public appearances. Last week, they appeared at an event at the former president's office to honor a Houston businessman and philanthropist with a Points of Light Award, a volunteer service award started by Bush.
The Associated Press contributed to this article
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