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

Stephen J. Hemsley returned as CEO after Andrew Witty's surprising decision to leave the insurance giant, which has seen its stock plummet recently.