Andrew Witty is stepping down as CEO of health care giant UnitedHealth Group in a surprise transition that follows a period of unprecedented turbulence, including the public killing of a company executive and financial woes that in recent weeks have only intensified.

UnitedHealth Group CEO Andrew Witty is stepping down from his post for personal reasons and the health care giant says it’s suspending its full-year forecast due to higher-than-expected medical costs

UnitedHealth Group said its Chief Executive Andrew Witty resigned as of Tuesday, and the company suspended its annual forecast due to surging medical costs, sending shares plunging nearly 18% to a four-year low.