Ampon Tangnoppakul, 62, who was considered a "prisoner of conscience" by Amnesty International, was convicted in November for sending text messages deemed insulting to the royal family in 2010. He was found dead in his cell on Tuesday.
"I have received an initial autopsy result that he had liver cancer and it spread to many other organs," Jongjet Aoajenpong, director of Police General Hospital where the examination was conducted, told AFP.
He said a tissue sample would be sent to a laboratory for confirmation.
Ampon, who became known in Thailand as "Uncle SMS", pleaded not guilty during his trial, one in a series under the kingdom's controversial lese majeste legislation, which critics say is used to stifle free speech.
His wife and political activists on Wednesday paraded the coffin containing his body through central Bangkok to the Criminal Court in the northern outskirts of the city.
The royal family is a very sensitive subject in the politically turbulent kingdom. King Bhumibol Adulyadej, 84, has reigned for 66 years but has been in hospital since September 2009.
Under the Thai law, anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count. Police are duty-bound to investigate complaints, which anyone in the country can lodge.