Piyachat Chanitsiripunya, 25, had suffered pains in the head after his girlfriend hit him with a hard object, and was later diagnosed with a form of mental illness, Pol Capt Kampanat Sithikaew of Mae Sai police station said. He had been treated at Suan Prung Hospital for five years.
Before his death, Piyachat isolated himself in his room and his cousin later found his body lying in a pool of blood, his throat slashed open.
An initial investigation pointed to mental illness as the cause of suicide. However, the family wants a deeper investigation to determine if a crime had been committed, Pol Capt Kampanat said.
His suicide comes on the heels of a macabre murder in Bangkok.
On Sunday morning a woman was arrested for killing and beheading her husband, then stashing his plastic wrapped body in a travel bag and throwing his head into a canal.
Phonsuri Diphaeo, 36, asked a security guard for help to move her belongings out of the room as she moved out of an apartment in the Bang Khunnon area of Bangkok Noi district in Bangkok.
The security guard became suspicious of the bags and contacted the police.
Police found a plastic bag with a male body inside minus the head and with its hands and right foot missing, reports said. Police later found the head in a plastic bag in a nearby canal.
Reports said Phonsuri had acted suspiciously since Saturday night, when she suddenly told the apartment office she wanted to end the lease. Early Sunday she had taken several plastic bags into the room.She was then seen to throw one of the bags into the canal.
Police said Phonsuri appeared delusional when questioned and told investigators she was a medium for a deity fighting the devil. She also admitted to using drugs.
Her husband, Prasit Sombunyanon, 47, had suffered from chronic fatigue and muscle weakness (myasthenia gravis) for some time and neighbours said he was often scolded, or even assaulted, by his wife, police said.
The police suspected that Phonsuri may be suffering from a mental illness, so she will be examined by psychiatrists.
In August a woman was arrested on charges of killing and possibly eating her two children in Mae Ai district of Chiang Mai. Police found the woman at her home with several body parts, apparently from children, near her.
The woman, whose name was withheld, had received treatment for mental illness in 2007 and had recently stopped taking her medication.
Mental Health Department deputy director general Dr Kiattiphum Wongrachit said the woman killed her children because she did not continue taking her medication.
The woman was charged with murder and sent to Suan Prung Hospital for treatment after she was deemed mentally unfit to fight her case.
Deputy Health Minister Surawith Konsomboon said recenty that the latest study by the Department of Mental Health concluded that around 20% of Thais, about one in every five, suffers some form of mental illness, the most common ailments being psychosis, anxiety disorders, depression and cerebral hemorrhage, or stroke.
However, only three million of them had been registered with hospitals nationwide last year. Of these, only 1.09 million had received regular treatment at the 17 psychiatric hospitals countrywide.
He added that the number of mental health patients is likely to grow each year.