A method to manufacture a throttle valve for an internal combustion engine comprising a valve body having a seat and a tube for the passage of conditioning fluid; an actuating device, which controls the rotation of a throttle plate; and a substantially uniform layer of a structural and heat-conducting resin interposed between the seat and the tube and applied on the entire available surface of the seat; the method comprising the steps of manufacturing the valve body provided with the seat by causing a first metal material to undergo a die casting process; applying a trace of the structural and thermosetting resin on the bottom of the seat; and inserting the tube into the seat so as to obtain a substantially uniform layer of the structural and thermosetting resin, which is interposed between the seat and the tube.