Upvote:3
You'll almost certainly clear US customs in Canada before getting on your flight, as Toronto Pearson has a US pre-clearance facility. So assuming your flight is a single booking you won't need to recheck.
Quoting the Toronto Pearson website on the matter (emphasis mine):
For the majority of U.S. flights, guests leaving Toronto will go through U.S. Customs in Toronto, which allows you to arrive in the United States as a domestic passenger.
I once had a layover in Miami going to South America from Canada and didn't have to redo security and immigration in the US.