〜としたら: if we suppose; assuming that

Asks you to suppose something that may not be true and then think about what follows.

Plain form + としたら (na-adjective and nouns often take だ: 本当だとしたら). Followed by the speaker's judgment or question, not a plain fact. Close to 〜とすれば and 〜とすると; 〜としたら is the most conversational.