Kathleen Sebelius has resigned as the U.S. Health Secretary after struggles with the rollout of the Affordable Care Act, it was learned on Thursday.

After five years in office, Sebelius handed her resignation in to President Barack Obama. On Friday, the president is set to nominate her successor, Sylvia Matthews Burwell, reports The New York Times. Burwell is currently the director of the Office of Management and Budget.

White House chief of staff Denis R. McDonough said of the choice of Burwell, "The president wants to make sure we have a proven manager and relentless implementer in the job over there, which is why he is going to nominate Sylvia."

Though officials say the 65-year-old Sebelius was not forced out, a longtime colleague was surprised by the sudden resignation, according to Businessweek. Kansas Insurance Commissioner Sandy Praeger said they had dinner recently and Sebelius didn't make any indication she was ready to leave her post.

She noted that the problems from the HealthCare.gov website shouldn't just be shouldered by Sebelius. "We can point fingers in a lot of different directions. I don't think it's fair to point them at her, solely."

Sebelius' resignation comes at a time where it appears that perhaps a corner has been turned when it comes to the ACA, or at the least, the problems appear to be over for the website. The Obama administration has touted that, in all, 7.5 million people have now signed up for healthcare coverage.