"Today I spent the whole day undergoing necessary medical procedures," he added.
Dmitry Kovtun, Lugovoy's business partner, is also in hospital with suspected radiation poisoning.
Mario Scaramella, an Italian contact of Litvinenko, has undergone treatment in London for the effects of similar contamination.
Medical notes
Interfax said its information on Lugovoy's condition came from his medical notes. "Disruption in the functioning of some organs affected by radiation nuclides has been found (in Lugovoy)," it quoted a source as saying.
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Meanwhile, police in Hamburg said they planned to examine Kovtun's apartment in the city for traces of polonium 210, following media reports that the Russian businessman had flown from Hamburg to London.
The statement did not say when Kovtun was last in Hamburg.
Lugovoy and Kovtun met Litvinenko at London's Millennium Hotel on November 1 for what Lugovoy described as a business meeting.
He has denied any part in Litvinenko's poisoning and offered to help police.
British detectives continue to work in Moscow as part of their investigation into the murder of Litvinenko, a British citizen.