The family of an 18-year-old cadet in the Thai military is demanding answers after his body was returned without its brain, heart and other organs, following an autopsy investigation into his sudden death.

The army said Pakapong Tanyakan died of a sudden heart attack, but did not release details of the autopsy and the family remained suspicious.

When his body was returned, the family held a fake cremation and secretly had a second autopsy performed at a private hospital, according to local news reports.

"It turns out when they opened up his skull, there was only tissue paper," said Supicha Tanyakan, the dead cadet's sister, as quoted in the Bangkok Post.

Pakapong Tanyakan's heart, stomach and bladder were also missing.

The second autopsy found both collarbones broken, a fractured fourth rib and internal bruising on the right side of his abdomen.

The injuries were not mentioned by army doctors after their autopsy.

"His body had no signs of injuries at all, so we partially removed some of his organs, like his brain and heart, for detailed examination," said Lieutenant Colonel Narut Thongsorn, a senior doctor at the Army Institute for Pathology, as quoted by The Nation newspaper on Tuesday.

The ABC has not seen either autopsy report.

Pakapong Tanyakan's family mourn his sudden loss and the unexplained condition of his body. ( Facebook: Sukanya Tanyakan )

Army history of hazing and recruit deaths

Thailand is a highly militarised nation and is currently run by generals, after a coup in 2014.

Each year a lottery for young males decides who gets drafted to duty, and the junta has even introduced military-style training programs in kindergartens.

Thailand's armed forces have faced scandals in the past involving hazing rituals and deaths of recruits.

In April, a 22-year old conscript died while being detained at the 45th Circle Military Camp in Surat Thani for disciplinary breaches.

According to Human Rights Watch, Yutthakinun Booniam's last words to his mother in hospital were: "I was beaten up. It hurt so much."

There are inconsistencies in the Thai army's explanation of Pakapong Tanyakan's death. ( Facebook: Sukanya Tanyakan )

Investigative news outlet Prachathai reported at least eight deaths of recruits in the last eight years from torture, punishment or unexplained reasons.

At a press conference in Bangkok on Tuesday, the officer in charge of the cadet school denied Pakapong Tanyakan's sudden death was linked to a punishment three months ago.

On 23 August, the 18-year old was among cadets caught using a forbidden pathway and was forced to sit with his legs above his head in a sauna room for several hours, according to his sister, Supicha Tanyakan.

"He fell unconscious and demonstrated low vital signs that day," Ms Supicha told Thai media.

On 16 October Pakapong Tanyakan participated in school activities but had quickened breathing and went to the first aid room, according to Major General Kanokpong Channuan, the commander of the cadet school.

He fainted on October 17 and was sent to a hospital, where he was pronounced dead, said Major General Kanokpong.

Different guidelines for autopsied organs in US, Australia

Australian forensic centres do sometimes retain organs for further examination, but only after extensive consultation with the family.

That would include a clear explanation about why the organ was being kept and sometimes arrangements with the family for the body part to be buried or cremated in a separate ceremony.

However, in 2015 a US court ruled that brains don't have to be returned inside bodies after an autopsy.

The case came to light when friends of a teenager killed in a car crash saw a brain in a jar with his name on it during a school trip to the morgue.

A lower court initially awarded the family $1.3 million but the Court of Appeals overturned the ruling.

"They had us bury our son with nothing in his head — it seems like they don't even have a heart," said the victim's father, Andre Shipley, according to the New York Post.