Jan 10 (Reuters) - Insurer American International Group said on Tuesday Chief Financial Officer Shane Fitzsimons will take a temporary medical leave of absence, effective immediately. Global Chief Actuary and Head of Portfolio Management Mark Lyons will return to his former role as AIG's CFO on an interim basis, the company said. (Reporting by Uday Sampath in Bengaluru; Editing by Devika Syamnath)