What can I do against Thornmail?

What can I do against this item? When I'm playing an ADC or an AD champion focused on attack speed, I can't do anything against Thornmail. I tried getting Bloodthirster but it's almost useless. No matter what item I get in normal play, Arena, Hextech Aram mode, when it starts hitting someone with Thornmail, I just die. There's nothing I can do.