BOSTON — Doctors evaluating Teresa Heinz Kerry, wife of U.S. Secretary of State John Kerry, reported improvement in her condition Monday, according to the State Department, but few details were being disclosed about her illness.

Heinz Kerry, 74, who is also heir to the Heinz ketchup fortune, was flown to Massachusetts General Hospital in Boston on Sunday after first being brought by ambulance to a hospital on the Massachusetts island of Nantucket, where the couple has a home and often spends time during the summer.

Heinz Kerry showed symptoms consistent with a seizure, said a person in close contact with the family who was not authorized to speak publicly about Heinz Kerry’s condition and spoke on condition of anonymity.

Doctors upgraded Heinz Kerry’s condition from critical to fair Monday after conducting tests and she was undergoing further evaluation, Glen Johnson, a spokesman for Kerry, said in a statement.

“The family is touched by the outpouring of well-wishes,” Johnson added.

Kerry was at the hospital with his wife Monday and was undecided about his schedule for the immediate future, according to the State Department. He had been expected to participate in a U.S.-China strategic and economic dialogue in Washington this week and there had been discussion of his possibly going overseas later in the week in an ongoing effort to restart peace talks between Israel and the Palestinians.

Heinz Kerry and John Kerry married in 1995. She is the widow of former U.S. Sen. John Heinz, who was killed along with six others in 1991 when a helicopter collided with a plane over a schoolyard in Merion, Pa.

In 2009, Heinz Kerry was diagnosed with breast cancer and underwent lumpectomies on both breasts.

“We are all praying that Teresa will recover quickly, she’s an amazing person,” said Philip Johnston, a former Massachusetts legislator and state Democratic party chair who has been friends with Kerry since the 1970s.

“She’s had such an extraordinary life, she has been so important to John over these past couple of decades,” said Johnston, who said he had not yet spoken with Kerry about his wife’s illness.

Heinz Kerry has been an active leader of two Heinz family philanthropic organizations that give millions of dollars each year to nonprofit groups.

Johnston said she has focused much of her energy in recent years on health care, an effort that began even before she was diagnosed with breast cancer.

“She is viewed in the philanthropic community as a pioneer in funding efforts that other people shied away from, including women’s health. She has used her leverage,” Johnston said, noting her funding of a large annual women’s health conference in Boston.

During her husband’s 2004 presidential campaign, Heinz Kerry drew both praise and criticism for her no-nonsense and often outspoken style, contrasting with Kerry’s more scripted approach.

She once told a persistent reporter to “shove it” and offered a rare apology after questioning whether Laura Bush, the wife of then-President George W. Bush and a former schoolteacher, ever had “a real job.”

Raised in the East African nation of Mozambique, the daughter of a physician, Heinz Kerry attended a university in Johannesburg. During her speech to delegates at the 2004 Democratic National Convention, she said while the university itself was not segregated, she witnessed the “weight of apartheid” around her and she marched with fellow students in a protest — ultimately unsuccessful — against the Higher Education Apartheid Act in South Africa.

The experience, she said at the time, taught her how special America was and how precious freedom was.