Shares of Northern Trust Corp fell by more than 9% to their lowest level in nearly a month on Tuesday after the custodian and asset manager forecast a bigger-than-expected drop in net interest income (NII) in the third quarter. Northern Trust's NII, the difference between what it makes on loans and pays out on deposits, is likely to fall by as much as 10% this quarter, Chief Financial Officer Jason Tyler told the Barclays financial conference on Tuesday. Like many financial institutions in the current high interest rate environment, Northern Trust is confronting growing financing costs as tries to hold onto deposits.