Kushner Says He Was Treated for Thyroid Cancer While in White House

“This was a personal problem and not for public consumption,” he wrote. “With the exception of Ivanka, Avi, Cassidy and Mulvaney, I didn’t tell anyone at the White House — including the president,” he wrote, referring to his wife, Ivanka Trump; two of his aides; and Mick Mulvaney, then the White House chief of staff.

Mr. Kushner was an assistant to the president and had a title, senior adviser, that only hinted at his omnipresent role in his father-in-law’s White House. Yet some veterans of previous administrations said that because Mr. Kushner was not an elected official, he had no obligation to disclose the information.

David Axelrod, the chief strategist in President Barack Obama’s White House, said that from his perspective, “If it didn’t affect his performance, it was a personal matter and he had no obligation to disclose it. Unlike any business entanglements or investments that might pose conflicts of interest, this was a personal, medical matter.”

In the excerpt, Mr. Kushner described concentrating on work and trying “not to think about the upcoming surgery or the unwanted growth in my body. When I did think about it, I reminded myself that it was in the hands of God and the doctors, and that whatever happened was out of my control. At moments, I caught myself wondering whether I would need extensive treatment.”

Mr. Trump was never known for his discretion about other people’s secrets. But while Mr. Kushner did not tell Mr. Trump of his diagnosis, he found out anyway, although he appeared not to have shared it.

“The day before the surgery, Trump called me into the Oval Office and motioned for his team to close the door. ‘Are you nervous about the surgery?’ he asked,” Mr. Kushner wrote.