An antithoron was a subatomic particle, the antimatter equivalent of a thoron.
In 2372, the USS Voyager bombarded the homeworld of the Sky Spirits with antithoron radiation, to decontaminate the reactive crust prior to the extraction of polyferranide. (VOY: "Tattoo")
In 2374, Voyager fired a level-8 antithoron burst towards a relay station in the Hirogen communications network. The burst weakened the containment field around the station's quantum singularity core, creating an intense gravity well that destroyed several attacking Hirogen ships. (VOY: "Hunters")