A reformer tube for producing synthesis gas by steam reforming of hydrocarbon-containing input gases is proposed where an outer shell tube is divided by means of a separating tray into a reaction chamber and an exit chamber, a dumped bed of a steam-reforming-active solid catalyst is arranged in the reaction chamber, at least one heat exchanger tube is arranged inside the reaction chamber and inside the dumped catalyst bed, whose entry end is in fluid connection with the dumped catalyst bed and whose exit end is in fluid connection with the exit chamber, the exit end of the heat exchanger tube is fed through the separating tray and opens out into a corrosion-protected inner tube which is disposed in the interior of the shell tube and is in fluid connection with a collection conduit for the synthesis gas product, and a gas-permeable thermal insulation layer is arranged between the inner wall of the shell tube and the outer wall of the inner tube.