Tie rod for straightening a door/panel/shutter which comprises: two engagement bodies (11) intended to be mechanically embedded in the thickness (S) of the door/panel/shutter in a non-centered manner with respect to a middle longitudinal plane (P) thereof, at two lateral edges (9, 10); and a rod (12) connected at the ends (13) to the two engagement bodies (11). Screw / nut screw means (19) are then provided for adjusting the length of the rod (12) subtended between the two engagement bodies (11). The rod (12) consists of a first and a second segment (12A, 12B) provided, for corresponding sections (15A, 15B) of their axial extension, with respective first and second shaped profiles (16A, 16B), which are mechanically couplable with each other with shape coupling for a variable shared section (17) in order to adjust the overall axial length of the rod (12) as a function of the height of the door/panel/shutter (2).