A 12th grader was found dead in his home on Monday night in Pathum Thani with a gunshot wound to his head, police said.
The schoolboy, identified as Pattarapong Luankua, 17, left a suicide note, police say.
Thanyaburi police were alerted of the teenager's death at 9pm at Charoenlarp housing estate in Tambon Lamphakood in Pathum Thani's Thanyaburi district. Forensic crime detection officers and rescuers of the Ruamkatanyu Foundation responded.
The teenager had a gunshot wound to his right temple, inflicted by a .22 rifle.
The suicide note, which was found near his body, offered apology to his parents for having to end his life. The letter said the boy had an urge to end his life four years ago and it worsened after his pet cat, called Lucky, disappeared.
The boy said in the letter that he lost all strength to do anything after "Lucky" disappeared.
In the letter, the teenager also apologised to his teacher for being absent from classes often, saying he had to be absent to seek a reason not to kill himself.
His sister, Chutisara Luankua, 22, said her brother did not go to school on Monday while she and her mother when to work. When they returned home, they found him in the toilet room on the second floor of the house.
Chutisara said her brother became depressed after his cat disappeared over a year ago.
The body was sent for a post mortem at the Thammasat University.