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.

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. 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.

Andrew Witty — who became UnitedHealth Group’s CEO in 2021 — is stepping down from the top post due to personal reasons after years leading large health care companies. During the tail end of Witty’s time as CEO of UnitedHealth,