Earlier, Turkish media reports said the man had taken doctors and other staff hostage at Istanbul's Cerrahpasa hospital.
But Zekayi Kutlubay, the hospital's chief physician, said the police officer was alone in the room and did not hold anyone hostage. He said police were trying to convince him not to kill himself.
"He is alone in the room," Kutlubay told reporters outside of the hospital earlier. "There is no hostage crisis."
Police later told The Associated Press that the 30-year-old man, who had been receiving psychiatric treatment at Cerrahpasa for the past two years, was taken to Istanbul's main psychiatric hospital.
Kutlubay said the man was armed with a gun, even though the police force had earlier confiscated his gun because of his psychiatric problems. It was not immediately known how he obtained the weapon.
Turkey is on edge following a spate of deadly attacks, including an armed assault at an Istanbul nightclub during a New Year's celebration that killed 39 people.