A teenage male was in police custody on Wednesday, after allegedly trying to kill himself, as police released more details of the murder of a young woman about to enter university.

Police detained the suspect, 18, only hours after the woman's bound, bagged and handless body was found among rubbish beneath the Udon Ratthaya expressway in Pathum Thani on Wednesday morning.

A motorcyclist had been seen dumping something there about 5am by a car garage owner. When questioned, the motorcyclist had replied it was a "thing he loved".

A source said the suspect was apprehended at a housing estate in Pathum Thani and had tried to take pills to end his own life. Police took him to a hospital.

Police named the suspect only as Thanakorn, 18, of Bangkok, and the victim as Waranya, also aged 18, of Uttaradit.  Both were about to enter university.

A police source said the suspect's elder brother, who lives in the same house, told police he heard his brother arguing with his girlfriend inside their room during the night. He shouted out at them to stop and they fell silent.

The elder brother said when he and his girlfriend left for work about 6am on Wednesday he noticed his younger brother’s motorcycle had some fresh red stains on it. He did not know if it was blood or not.

About noon, he received a call from his mother, asking him to return to the house and check on his brother. When he arrived, the door to Thanakorn’s bedroom was locked. He broke in through the door and saw his young brother lying naked and having convulsions. He believed his brother tried to commit suicide.

The suspect was unable to give police his side of the story. He was still in a hospital in Pathum Thani, according to the police source.

The suspect's father told police his son had a mental problem.

The suspect and the victim had lived together for three years.

The suspect allegedly left the house to dump the body around 5am following an argument with his girlfriend.

There were blood stains in the bathroom in the victim’s bedroom. Police had also found a blood-stained machete.

The woman's body was found wrapped in a bedsheet and bound with three black belts worn by university students tied around her knees, waist and neck. From the waist down, her legs were fastened with coat hangers.

She had slash wounds to her neck and both her hands had been cut off. Her hands were missing.

The owner of a car garage beneath the expressway said he saw a man riding a motorcycle dump something beneath the expressway about 5am.

The motorcyclist first attempted to cover the object with grass and then picked up bags of garbage lying nearby and emptied them over the spot before riding his bike away.

About 8am, the garage owner's wife saw the garbage scattered over the area, went to look closer and saw what appeared to be a human foot. She immediately called police.