(CNN) -- Teresa Heinz Kerry, wife of Secretary of State John Kerry, has been upgraded from critical to fair condition a day after she was hospitalized with symptoms "consistent with some kind of seizure."
Heinz Kerry, 74, underwent tests at Boston's Massachusetts General Hospital on Sunday night and Monday morning, Kerry spokesman Glen Johnson said, before her condition was upgraded.
A source close to the family has told CNN she exhibited symptoms "consistent with some sort of seizure."
While leaving his home in Boston Monday evening, Secretary Kerry told reporters that his wife was improving and that the family hoped to have good news to report Tuesday. He thanked everyone for their well wishes.
Johnson earlier said John Kerry, Heinz Kerry's son and other family members have been at the hospital since she fell ill.
Heinz Kerry became ill Sunday and was taken by ambulance to Nantucket Cottage Hospital, accompanied by her husband, Johnson said.
Once her condition was stabilized, she was transferred to Massachusetts General.
Heinz Kerry was in Nantucket, where the family has a home.
"The family is grateful for the outpouring of support it has received and aware of the interest in her condition, but they ask for privacy at this time," Johnson said in a statement.
Born in Mozambique, Heinz Kerry worked for the Trusteeship Council of the United Nations in the 1960s when she moved to the United States.
Her first husband, Sen. John Heinz of Pennsylvania, died in a plane crash in 1991.
Heinz Kerry spoke to CNN's Larry King in 2010 about her battle with breast cancer.
He asked her if she was scared.
"I wasn't scared because I'm very analytical about things like this," she said. "I'm kind of, I guess, a little fatalistic."
CNN's Jamie Crawford contributed to this report.