Javanfekr, a media adviser to President Mahmoud Ahmadinejad, was admitted to a Tehran hospital for a heart condition he has struggled with for a year, his son Ali Akbar Javanfekr told the ISNA news agency.
Javanfekr had been released at midnight on Wednesday for a four-day parole to seek "medical treatment, after which he is committed to return to prison," the government-run Iran newspaper said in a short report.
Javanfekr, who heads the paper and the official IRNA news agency, was arrested in September and sent to Evin for six months after being convicted of publishing material offensive to Islamic codes and public morality.
He told the Fars news agency on Thursday that he would use the four days to recover so "he could manage the three months remaining of his sentence," adding that his priority was getting "medical treatment."
"Going on (prison) leave is a normal phenomenon for all prisoners... and it could be because of illness," his lawyer Ghahreman Shojaei told ISNA.
Javanfekr has long been targeted by hardline judges and ultra-conservative figures who see him and the president as trying to undermine religious principles.
His arrest, at the time Ahmadinejad was delivering a speech at the annual UN General Assembly in New York, sparked infighting in the regime.
In October, Ahmadinejad planned a visit to Evin, where most of the inmates are political prisoners, to meet Javanfekr.
But the judiciary, controlled by hardliners close to supreme leader Ayatollah Ali Khamenei, denied the president access to the jail.