Is romex wire thhn?

No, Romex wire is not THHN. Romex is a brand name for non-metallic sheathed cable, which typically contains individual conductors insulated with PVC (polyvinyl chloride). THHN (Thermoplastic High Heat-resistant Nylon-coated) is a specific type of insulation used for individual conductors, often found within conduits or other cable assemblies, but not typically as the outer jacket of Romex.