Pourandarjani worked at the facility once a week as part of his mandatory military service.
His death on November 10 was first reported by the opposition and later confirmed by Iranian authorities.
But recent reports on opposition websites have suggested the doctor was killed because he knew the conditions of a number of alleged torture victims at the prison, including Mohsen Rouhalamini, the son of a prominent political figure.
Opposition website Mowjcamp said Pourandarjani was interviewed by the parliamentary committee investigating the alleged abuse.
He told them Rouhalamini was brought to him at Kahrizak "in a dreadful state after being subjected to extreme physical torture. He was in a critical state", the website reported.
|Ali Khamenei order Kahrizak to be closed amid the torture allegations [GALLO/GETTY]
It said Pourandarjani had told the committee that after the youth's death, "officials in Kahrizak threatened that if I disclosed the causes of the wounds of the injured at Kahrizak, I would not be able to live".
Hanif Mazroui, a reporter for the opposition news website Roozonline, said he was contacted by a doctor earlier this week who told him Pourandarjani had committed suicide.
The doctor, a colleague of Pourandarjani, later called Mazroui back and said the cause of death was "suspicious, and [authorities] are not making it public", Mazroui told the Associated Press news agency.
The authorities had initially announced that Pourandarjani had a heart attack in his sleep, but later claimed he died of poisoning.
The office of Abbas Dowlatabadi, Tehran's general prosecutor, said on Monday "preliminary autopsy revealed he did not die of poisoning".
The doctor's father, Reza-Qoli Pourandarjani, said he did not believe any of the causes given so far by the government in his son's death, but he did not accuse the authorities of killing his son.
The body was handed over to the family, and was buried in the city of Tabriz on Sunday, but Pourandarjani's father said he never saw the body because he was too grief-stricken.
In August, Iranian officials acknowledged that some protesters detained in the crackdown on protests in June - which followed the re-election Mahmoud Ahmadinejad, the president - were abused in prison.
Iran's prosecutor-general said there had been "violations and carelessness" at Kahrizak prison, while Esmail Ahmadi-Moghaddam, Iran's police chief, admitted prisoners had been beaten by their jailers.