Alberto Zangrillo said he saw Berlusconi on Monday and decided to transfer him to a nearby hospital because he did not think it prudent to bring him to Italy, which is some 10 miles from the small city-state.
Italy's ANSA news agency first reported the news. It was not immediately clear if the health emergency had been overcome, but Berlusconi later said in a statement that he was "in good health."
"Due to the prudence of my doctors I had to go to hospital for some checks," he said, adding that he was in contact with his staff as Italy deals with the political crisis threatening to bring down the cabinet led by Giuseppe Conte amid the coronavirus pandemic.
The 84-year-old media tycoon underwent major heart surgery in 2016 and has also survived prostate cancer.
Last September he was hospitalized after contracting coronavirus. He later told reporters that the virus had posed "the most dangerous challenge" of his life. Berlusconi has spent much of his time recuperating at his daughter Marina's house in Provence, southeast France.
Berlusconi also remains head of the opposition Forza Italia party. His illness comes as Italy has been hit by political chaos after a junior coalition party quit the government, depriving it of a majority in parliament.
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Newspapers have speculated that some Forza Italia lawmakers might agree to desert party ranks and help prop up Prime Minister Giuseppe Conte to prevent early elections.
Shares in Mediaset, the broadcaster controlled by the Berlusconi family, jumped as much as 2.9 percent on Thursday after initial reports of his latest health scare.
Traders cited speculation about potential ownership changes at the group if Berlusconi's condition worsened.
Mediaset has been at the centre of a legal battle between Berlusconi's family holding Fininvest and its second-largest shareholder, French media giant Vivendi, over the past five years.