The News24 website said Pistorius was taken from Atteridgeville prison in Pretoria to hospital in an ambulance after complaining of chest pains.
Do you think that a man so young could possibly have suffered a heart attack while in prison?
Pistorius was treated at the hospital's designated prison ward and discharged at 2pm.
Maistry declined to give details of Pistorius' medical complaint, citing department rules preventing the divulging of information about offenders.
The former athlete has been behind bars for a year and is now serving a six-year sentence for killing his girlfriend, Reeva Steenkamp, in 2013. "What I am aware of is that he was this morning taken to an outside hospital for a medical examination ..."
Earlier, it was reported that Pistorius would be released from the hospital and go back to prison today. A year ago he was taken to the hospital for treatment to cuts on his wrists, which prison authorities said he sustained after falling in his cell.
Pistorius - nicknamed Blade Runner due to the carbon fibre prosthetic limbs he used to run - shot dead model Reeva on Valentine's Day 2013.
Pistorius was convicted of murder in 2015 after an appeal by prosecutors against an initial manslaughter verdict. Pistorius claimed he mistook his girlfriend for a nighttime intruder. Pistorius' appeal to overturn the upgraded murder conviction was denied.
A year ago he injured his wrist while in his cell.
In November Pistorius moved to a new jail in South Africa after demanding one with better bath tubs. South African prosecutors are in the process of appealing Pistorius' six-year sentence with the intention of having him serve a maximum 15-year sentence.