By MICHELLE CHAPMAN, Associated Press Business Writer

UnitedHealth CEO Andrew Witty is stepping down for personal reasons and the nation’s largest health insurer suspended its full-year financial outlook due to higher-than-expected medical costs.

Chairman Stephen Hemsley will become CEO, effective immediately, the Minnesota company said.

Hemsley was UnitedHealth Group CEO from 2006 to 2017. He will remain chairman of the company’s board. Witty will serve as a senior adviser to Hemsley.

“Leading the people of UnitedHealth Group has been a tremendous honor as they work every day to improve the health system, and they will continue to inspire me,” Witty said.

Witty joined the company in 2018 after serving about nine years as CEO of the British drugmaker GlaxoSmithKline. He was named UnitedHealth’s CEO in February 2021, replacing Dave Wichmann.

UnitedHealth became one of the nation’s largest companies under Witty’s leadership. Total revenue topped $400 billion last year, a 55% increase from the $257 billion UnitedHealth brought in the year before Witty became CEO.

Shares of UnitedHealth rocketed higher under Witty, up 60.5% since he took the company’s top job.

Yet in the past five months, that stock performance reversed sharply, coinciding with the fatal shooting of company executive Brian Thompson in front of a New York City hotel late last year.

The company has wrestled with the attention on Luigi Mangione, who was indicted last month on a federal murder charge in the killing of Thompson.

The case has captured the American imagination, setting off a cascade of resentment and online vitriol toward U.S. health insurers while rattling corporate executives concerned about security.

UnitedHealth cut its 2025 forecast last month following its first quarterly earnings miss in more than a decade. Shares of UnitedHealth, which have plummeted 38% since the deadly Dec. 4 ambush of Thompson in midtown Manhattan, fell more than 8% before the opening bell on Tuesday.

UnitedHealth said Tuesday that it suspended its 2025 outlook as medical costs of many Medicare Advantage beneficiaries new to UnitedHealthcare were higher than expected.

More than 50 million people have health insurance under UnitedHealth Group Inc. It also has a large pharmacy benefit manager that runs prescription drug coverage and a growing Optum segment that delivers care and provides technical support.

UnitedHealthcare is the nation’s largest provider of Medicare Advantage plans, with more than 8 million customers. Those are privately run versions of the federal government coverage program mostly for people ages 65 and older.

AP Health Writer Tom Murphy contributed to this report.