UnitedHealth Group said CEO Andrew Witty is stepping down for “personal reasons” and suspended its 2025 forecast, sending shares of the healthcare giant tumbling nearly 10%.

Witty will act as a senior advisor to his successor, Stephen Hemsley, who served as UnitedHealth Group’s CEO from 2006 to 2017.

Witty oversaw a tumultuous last year for the company, which grappled with government investigations, a historic cyberattack, higher-than-expected medical costs and the torrent of public blowback after the murder of UnitedHealthcare’s CEO Brian Thompson.