Police said the body of Christina Annesley, 23, was discovered inside a bungalow room at a resort on the eastern side of the island on Wednesday.
There were no immediate signs of criminal activity but her body will be sent to the mainland for an autopsy.
"There was no trace of a fight and her body has no wounds," local police officer Lieutenant Colonel Napa Senathit told AFP by telephone, adding that investigators discovered three kinds of medicine in her room, but no recreational drugs.
The autopsy is not likely to be carried out before Sunday as officials have to wait for a ferry to the mainland.
"There is no ferry today so we will send her body on Saturday night to arrive on the mainland Sunday morning," Lieutenant Colonel Napa said.
Many of Annesley's final tweets -- under the handle @chrstinadarling -- spoke of the beauty of Koh Tao's white sand beaches and azure waters which are popular backpackers and divers.
"In awe of how beautiful this place is," she wrote on January 19, posting a picture of one of the island's many beaches.
She also made reference to taking both Tramadol, a painkiller, and Valium, an anti-anxiety medication, in the days leading up to her death.
British daily The Telegraph said Christina's mother Margaret had posted on her daughter's Facebook page saying she had died of "natural causes". The Facebook page appeared to be inaccessible Friday.